Oversized hamster?

might just be bigger

Some thoughts come to mind, are you sure it is a male? How long have you had it? If it is a female newly added to your family it may have spent some time in the pet shop with a male hamster and it could be pregnant. Did it go from slim and active to fat too quickly? Is it behaving the same as before? If there have been sudden changes consider taking it to the vet to rule out health issues, (my hamster became very fat unexpectedly fast, her eating habits and behavior changed -- she had a tumor.) If none of this seems to be the problem then look at the diet. Those seed filled hamster foods make hamsters want to eat mainly less healthy parts of the food. Try using a rat/mouse pelleted diet and give just a teaspoon or so seed treat hidden about the cage as a treat it has to work to find. Use a ball and wheel, too. Hamsters need to run!

what can i say that hasn't already been said and i had to laugh. everyone is right including the possibility that it may be lazy. i had a wheel for my mice and the little fat one would run it all day almost neurotically but still kept getting fatter and fatter it may be genes.exersize is always the key to good health

Try feeding him less, and maybe get one of those balls and have him run around the house in it for a while for extra excersize to loose some of the weight.

The problem might be the food. But how big is the cage? If the cage is to small then it won't have any room to run around and exercise. If not then try feeding it less food. That might help. Good Luck!