NO, not really. Once a planet has formed then its mass is pretty constant. The only addition is by meteoritic debris which is about 200 tons per day.... that may sound like a lot but when you note that the mass of the earth is 6,000,000,000,000,000,000,000 tons then us see that even over all of geologic time of 4.5 billion years, the addition of mass has been about less than 1,000,000,000,000 tons. That is a VERY VERY VERY small fraction of the mass of the Earth!

They do!But very, very slowly: our solar system is full of material left over from its formation (mainly, asteroids and comets). As these objects fly around, sooner or later they hit a planet, increasing its mass. This process occurred relatively quickly when the solar system began to form, but as the planets started to accumulate all of the material near their orbit, the process slowed down. It still occurs today (every time an asteroid hits the Earth, the Earth's mass increases a little bit), but the rate of growth of the size of the planets from these contributions is negligible.

During the birth of our solar system, planets grew in size for many millions of years as asteroids, comets, and dust collided to form the planets. However, over time the solar system became more organized, and collisions with planets became much more rare. The planets reached their modern masses quite a long time ago (sometime around 4.5 billion years ago). Since then, the planets have both lost mass (mostly gases in the upper atmosphere), and gained mass (accumulation of meteorites and dust). In general, these additions and subtractions of material have not led to a significant change in planet size.

The other factor affecting planet size is temperature. As our planet and others like it radiate away their internal heat, they begin to cool and contract. You can imagine earth as a balloon that you take from a warm house and set out in the cold. As the internal balloon heat radiates away, the balloon will shrink. Similarly, the planets are expected to shrink as they cool off in the vacuum of space. Again, this is a very slow process (taking billions of years), and the magnitude of change is probably difficult to detect.

Generally speaking, no - except during planet formation. Every time an asteroid or comet hits a planet, generally speaking the mass of the asteroid or comet is added to that of the planet. However, because this happens, planets tend to "gobble up" most of the asteroids and comets in a solar system early in the solar system's history, so by now the mass remaining in the solar system contained within asteroids and comets amounts to a pittance compared to the significant planets in the solar system.