Really? They do that? I haven't seen it, so I had no idea. I've had a few litters grow up with a runty baby. In some cases, the runt died somewhere in the fuzzy/hopper age range, but appeared fully intact, so I assumed it just had something wrong with it and died, rather than being killed. Right now I have one really tiny baby in my grow up cage that seems almost perfectly healthy other than her size. Her fur does seem a little thin so I'm just watching to see what happens out of curiosity mostly, plus she's pretty cute. I've also had several times when one litter is born, and another is born maybe a week later, and that doesn't seem to cause problems either.
Maybe my mice are just stupid about culling off their weak offspring.
I probably should have explained a little more. This is a 1.4 colony, so they've really just had about one round of babies. They have pretty much used up their chances now, though; it is just a matter of waiting for some replacements to grow up.