Is this a glitch? Or what am I missing?

Hmmm, I guess that depends on what the game considers overfeeding. I don't usually wait until they're crying before feeding them, but I do usually feed them every four sim hours or so. I just always thought if the babies weren't hungry, they'd reject the feeding, I've also noticed they feed for less time if not as much time has passed. I do breastfeed them 90% of the time too, not sure if that makes a difference.

If that is the case I sort of have an issue with the game doing that. We know in real life that studies show chubby babies actually end up growing into thinner adults, not to mention I'm pretty sure live babies feed as their hunger dictates, i.e. when they aren't as hungry they simply don't eat as much. If the game is encouraging players to wait to feed their babies when they're starving, or else making them fat, that seems wrong to me.