Would you put it on a diet?
Edit:Come to think of it what the fuck would a thin hamster look like?
Well, I don't have hamsters but I do have a few pet rats and occasionally they have chubbied up. I don't put them on a diet as such (monitoring a strict regime of celery and exercise ;) ) but I do take the fatty or sugary stuff out of their food for a while. They eat pretty much what we do (if I tried to give them "rat pellets" they'd revolt and murder me in my bed :gundge: ) so they eat a wide variety of stuff.
As for what a thin one would look like - I spose it'd be like humans - fat, then toned, then a little thin, then emaciated. You have to choose what is the healthiest size for the animal, regardless of fashion (regardless of the species, too, I guess)