Live super-breeders

Well, I have a growout tank for plants in which I also have some Guppies. These guys are probably into their 6th or 7th generations now, and it seems to me that they control their own population by the number of their own young that they eat.
If There's only a few adults in the tank, then you can see lots of fry running around, but when the fry grow up and the tank starts to get crowded, then I don't see any fry, so I'm assuming they are eating them. Then as the older ones die off and the population goes down, I start seeing fry again. Make sense ??
Anyway, that's my theory, and I'm sticking to it.