There are various types of plants, land and aquatic. For the most part, plants grow better (which is what I think you mean by bigger, because all plants are different sizes) in soil. Plants need sunlight, water, nutrients from the soil, and air to grow the best. When they are just in water, they aren't exposed to as much sunlight as they would be if they were on land in soil, they don't get as much air, and they don't get the nutrients from the soil. Plants do grow in water, but they grow the best planted on land in soil where they can get soil, sunlight, water, and air.

In general, plants need more than just water to grow big and healthy, although water is a good start, and seeds can usually be "germinated" with just water. Germinating is getting the seed to sprout. You can try this at home by putting a seed in a ziplock pouch with a moist paper towel and placing it in the dark. Most seeds will germinate in a few days if they are fresh.

After the seeds are sprouted, you usually want to put the sprouts in soil because soil provides additional nutrients. Most plants prefer a certain chemical environment to thrive. There is an entire field called soil science that involves a lot of chemistry and is extremely important for agriculture.

There is a type of agriculture that is soil-free, and that is called "hydroponics." In hydroponics, the plants are mostly grown in water, however nutrients and minerals are added to the water to ensure that the plants can grow healthy.

That's a tricky question. Plants need 3 basic things to grow: Sunlight, water and nutrients. Without one of these, the plant will not survive. Plants can be grown in water (without soil), but only if we add nutrients to the water (fertilizer). If it was pure water, then the plants would not survive. If you tried to grow a plant in soil without water, then the plant would not survive also. So basically, if you give plants unlimited water, light and nutrients then they will grow best. Different plants have different requirements, but that is the general formula.