I got two ferrets, both males and from the same litter, from my local pet store about two months ago. It seems only one of my ferrets is growing. Link is still the same size as when I got him but Loki is noticeably larger. He doesn't seem to be overweight, just larger. His coat is also fluffier than his brothers. Link doesn't seem underweight either and he eats more than his brother. Could it be a health issue or is it normal for some ferrets to stay small?

If you are concerned, certainly get him checked out for health issues, however some ferrets are big and some are small, there is lots of size variation in ferrets and it's not unusual to have different sizes in one litter :)

Ferrets coming from pet stores....its hard to tell if they are from the same litter.  Your little one may been the runt of the litter....maybe this is why he is so little.

I purchased two hobs from private breeder.  One of them was the runt of the litter.    Here are some baby pictures of him....as you can see he was the smallest.






Now he is a big boy