People who breed feeders want the biggest litters they can get from healthy females. So they are more likely to hold back females who give huge litters. Pet breeders focus on the look and color more than the litter size.

Both should be focused on the health, of course, but a pet breeder often doesn't even want large litters.

I get the best litters from rats from feeder sources, although I haven't needed to add in outside stock for a long time now. I've got my colony culled to the traits I want, although I'm refining them to the cream of the crop AND the biggest litters now.