Sometimes a baby is lighter smaller than it should be what is this called?

Usually, when a baby animal is smaller and lighter then the rest of his/her siblings they would be called the "runt" of the litter. Although, when they grow up they will be about the same size but when theyre babies, they are just a big smaller, but sometimes if they are TO SMALL it could be dangerous. So , treat the baby "runt" animal of the litter the same, but dont expect it to adapt as quickly as the others. I have two dogs that were the runt of the group and theyre now as healthy as ever, and the perfect size. It took them a little bit longer (about a week) to adapt like the others, but now, theyre perfect.