In my experience, cats that have gone hungry (abandonment/mother's illness, etc) are more apt to overeat than those that haven't.

Still, kittens are voracious. I used to call Cassy and Sassy my "furred piranhas" due to their ability to attack the feedbowl in mid-air as I was trying to put it down for them.

Are they eating kitten-specific food? I've heard that they eat less of that (due to it's richness) than a kitten will of cat food. Yes, check with your vet. Good luck.