Which I don't have any complaint with, but the current setup seems both extreme and limited at the same time.
I'm presuming from the mention that planets are rated on a scale of 1-30 on one of the Encyclopedia articles that planets can start class 30 (I know they can start at at least class 27 from those two class 31 planets I have now).
A planet that does start at class 30, just with the three terraforming improvements that are in place now, can reach class 54 (30+3+3+18), and this is without considering any of the techs/improvements that haven't been implemented, or any wonders that will eventually be appearing.
A planet that starts at class 6 (which I have colonized before to get more research points) cannot improve past class 9 though, barring any wonders in the vein of the Crystalai, because only terraforming can raise it a full class, and the game appears to drop partial class improvements.
Class 1 planets, not that anyone would be colonizing them often, cannot be improved at all as it stands now, and anything that uses the current % improvement system that could do so would have to be at least a 100% increase, which would just make high class planets more extreme.
I wonder if you could actually roll over the population on a class 108 planet.