The pop growth bonus is additive, so perhaps you're upset for no reason at all...?
If you want to achieve a good popgrowth, then the first thing you should consider to aim for is 100% approval. That is, you select a planet that has one (or more) moral bonus tile(s) and you place your very best moral improvements on them, usually this will be the Political Capital.
If the planet has food bonus tiles, use them, because this is going to give you some puffer in time.
All other tiles will be filled up with Fertility Clinics.
Ofc this planet is going to produce much more population than could possibly be brought away by own constructed Troopships, so you need to have quite some planets in the vicinity which are producing these Transports.
Just to make myself clear, the bonus to popgrowth from 100% moral is so much better than a few Fert Clinics, but nevertheless it will multiply its own bonus with these, so to have both is the absolute best way to achieve awesome popgrowth, except Breeder SA.