As far as I know, popularity gives you a boost to population increase only after you surpass certain boundary conditions.
* planet's approval modifier is:
- 0 if approval is less than 21%. In addition, you lose 10% of your existing population per turn.
- 0 for approval between 21 and 40% (pop doesn't grow).
- 1 for approval between 41% and 75% inclusive.
- 1.25 for approval between 76% and 99% inclusive.
- 2 for 100% approval without the Super Breeder special ability or 8 for 100% approval with Super Breeder.
This makes it pretty pointless to try to keep planetary approval higher than those boundaries. (Why would I need 85% approval, if it gives me the same modifier as 75%, plus extra taxes?) I realize that different planets have different approval modifiers, but still... Wouldn't it make more sense to use a formula that makes the penalties/bonuses to change gradually in between those boundaries? This way every bit of approval would be useful.