Ah, ok. Due to the display I always thought it was a minimum of 2%, I never actually checked how much it was growing to see if it was different from displayed. Good to know.
Well then, my new suggestion is to incorporate the size of the planet and perhaps availability of water into the growth equation.
Maybe take the diameter divided by 10,000 (or some other number, you could use the diameter of earth if you wanted earth to be 1) and then divide the CurrentPop by that before using it, so that larger worlds could support the larger pop before getting into the lower growth rates, and thus be more desireable for colonization.
Something like this:
Growth Rate = 20 / ((CurrentPop / (Diameter / 10000)) ^ (1 / 3))
That way a 1 billion pop world Earth's size would grow at a rate of about 2.17, while 1 billion pop on Mars would grow at a rate of about 1.76. Or if you used the diameter of earth as the diameter divisor you would have 2% on earth and 1.62% on mars.
Presence of water could be just a simple modifier added in the same way as the governor bonus, like a straight 10% bonus for having water. Or you could do it based on the extent, like extent / 5. So mars would be a 2% boost for the 10% water while Earth would be a 14% boost since its 70%. If water already does something cool that I don't know about it may not be necessary, but I always thought having water should be a "Great! That planet's my next colony" sort of thing, since water is so necessary for life.
I hope you don't mind suggestions like these. When I make a suggestion on something I try to come up with some sort of formula or example if its relevant, and here since you've given me the formula for how it currently works I'm fiddling with that. I've known some people that don't like others meddling with their math, so if this is unwelcome just let me know.