I've noticed, on several occaisons, that some planets don't seem to grow very fast. I've actually been paying specific attention lately, and I've discovered that it's pretty simple--any planet of PQ 5 or lower "only" gains 1 million (0.001 billion) people per week, even with my +120% reproduction rate.
It's easy enough to transport more people in (especially with my bonuses in my most recent game), but I've never heard anything about this. Is it normal?