Size has distinct advantages and disadvantages. Large animals, as you say, need lots of food and space, and there are design complications with supporting the animal. However, larger animals are also unlikely to become prey for smaller animals, and larger animals (if warm-blooded) also lose heat slower than smaller animals (when you take geometry, if you haven't already, you'll see why).

So, there are advantages to being big. Whales are much bigger than we are so that other creatures in the sea cannot prey upon them and so that they retain their heat inside their bodies. We, likewise, are bigger than ants, and for the same reasons: an anteater is harmless (to us), but it's a deadly terror if you're an ant!

Being a big animal has both advantages and disadvantages. Big animals lose far less body heat than small animals, and therefore don't need to rely as much on consuming high calorie food. Big animals conserve body heat very efficiently, and often don't even need hair (an elephant barely has any hair). They also can use their size for defense, and can live in places that have large fluctuations in heat and cold because they monitor their own temperature so well.

For many, many millions of years, dinosaurs dominated the planet, and small mammals existed in relatively small numbers. The benefit/cost of being a large animal depends entirely on the current environment. Some paleontologists think that the reason that mammals thrived after the meteor that hit Earth and caused the extinction of the dinosaurs is because they were small enough to be able to hide from the aftermath of the meteor strike. In that case, being big would have been a very bad disadvantage after many millions of years of large size being a great benefit.

That's a great question. You're right that big animals need a lot of food, so they need a lot of space for that food to grow. On the other hand, there are some benefits. For one, you may be so big that no predators can kill you. Elephants do not have any non-human predators. Even their babies are protected by the herd. Sometimes predators get together to attack large animals, but they can usually only kill the weakest.

As any object gets bigger, the ratio of its surface area (its outside) to its volume (its inside) gets smaller. In other words, its inside grows faster than its outside. You can see this by calculating the surface area and volume of a cube with 2-cm sides and one with 3-cm sides. What this means to the animal is that it loses heat much more slowly so they are much more efficient with their food. Mammals and birds have to spend huge amounts of energy to make heat. Small animals like shrews and hummingbirds have to make so much heat that they can starve in only a couple of hours. Their hearts have to beat almost as fast as a heart can beat. Huge animals have very slow heart rates. So they need LESS food PER KILOGRAM even though they need more total food.

So if you were a dinosaur, would you rather be the biggest in the herd or the smallest?

Does your food source matter?
Does the environment matter?
Do your predators matter?

By the way, there were once dwarf elephants on islands in what is now Greece. Why do you think island elephants might be so much smaller than their mainland relatives?

Organisms don't stick around too long if they're "useless." In some cases being large probably has advantages. Being big, for example, decreases the amount of surface are an animal has (per unit volume), which means they lose less heat (per unit volume) than do smaller animals, which in turn means that they can survive on poorer quality food than can smaller animals.

Being huge also protects some animals from being eaten.

Large animals are not useless - This is attested to by the fact that big animals have repeatedly evolved over time from smaller ancestors in a really wide range of animal groups. Some reasons include that
- when you are big, you have a lower metabolic rate. Thus, while you need more food overall, you need less food per pound of you than if you were tiny
- when you are big, you have a bigger stomach. Particularly among herbivores, this means you can eat very low quality food and then digest it over a long time frame to gain nutrition. A small stomach cannot do this.
- When you are big you do not particularly have to worry about predators
- When you are big you have a reduced surface area relative to your volume
- so you lose less heat from your surface. This is good in cool places(polar environments; the ocean).

That's a good question, and I have both a short and a long answer for you.

The short answer is that animal species only ever evolve into forms that work best in their environment. That's the only reason any species ever evolves into any other species. "Working best in their environment" really means reproducing the most. Living longer, finding more food, being stronger or faster, or being better at hiding from predators all only matter because these things allow an animal more time to reproduce. Producing more offspring is, ultimately, the only evolutionary advantage that matters.

Sometimes a species will evolve into a new species not because the new species is better able to reproduce, but just because the new species is not WORSE at reproducing.

The longer answer I have is a list of possible reasons why being big like a dinosaur or a blue whale might allow some animals to reproduce more. In the ocean, there are fewer disadvantages to being huge, since you can float in the water and let it support your weight instead of having to have your legs support your whole weight. The food that blue whales eat is extremely abundant--there are literally tons of it everywhere. Being unable to find enough food is often a thing that stops animals from getting bigger, but large whales don't have that problem. Also, the huge size of a Blue Whale protects it from most of the ocean's predators, since few of them are big enough to kill and eat them. There might be other advantages to being that big, or it might be simply that there aren't many disadvantages.

Dinosaurs probably had similar reasons for evolving into such big species,but scientists don't really know. The ones that lived on land didn't have the ocean to help support their weight, but the warmer climate probably made it easier for them to maintain their body temperatures (whether they were cold-blooded or warm-blooded). The body temperatures of small animals change much more quickly than those of large animals. Generally, though, I think the fact that there are such huge marine animals but so few huge land animals means that being huge on land doesn't work as well as being huge in the ocean.

So, remember the short answer: all species only ever evolve into new species if doing so makes them better at reproducing (or, at the very least, if it doesn't make them worse at reproducing). Also remember that individual animals don't evolve, animal SPECIES evolve--and that evolution happens over many generations. Keep those things in mind, and you'll be on the right track in thinking about evolution! And keep asking great questions!

"Useless" is a little harsh, don't you think? Things evolve to be big because that's one of the only sure-fire ways to avoid getting eaten. The blue whales are larger than the largest predatory animals in the sea:

killer whales and white sharks. Sure, if enough killer whales and white sharks ganged up on a humpback whale it might not be able to escape predation, and humans have certainly evolved ways of killing blue whale with guns and harpoons, but even then, it still involves a huge struggle.

Size definitely has its advantages. In fact, you could have asked why aren't most animals as big as blue whales? Most animals are limited in their size by something. Insects, for example, are pretty much limited in their size by the fact that they don't have lungs. Most insects rely on oxygen diffusing into their bodies through tiny holes in their sides,near the legs. They can't just take deeper breaths to get more oxygen. In prehistoric times when there were lots more plants, there was actually more oxygen in the atmosphere than there is now, and insects were bigger (imagine fending off four inch cockroaches and giant predatory centipedes five feet long and a foot wide!). Blue whales have an advantage in the ocean because water bears most of their weight, and they don't need to worry about having massive skeletons to support all the blubber, but the dinosaurs showed us that it was definitely possible to have skeletons strong enough to support lots of weight.

It does seem like many organisms in the past were larger: there are fossil penguins from Antarctica that stood almost 6 feet tall. On the other hand, humans have been getting taller since we first evolved, probably as our diet improved because of agriculture and tools for hunting. We may be 20 feet tall someday.