Why is it that in every game, small class planets cannot grow their population past a certain limit?

For instance, Mars, with two farms and population limit of 10.0b, will never, ever grow beyond 2.51b? Is there are normal reason for this?

I can use colony ships to keep building up the population to its maximum, so it seems that only the population GROWTH is permanently 0.

Other planets do this a seemingly different numbers, but they are usually planets of class 5 or less.

Can anyone help me? I have the latest patch already.