Like any animal , hams can overeat. My biggest unfounded fear though when getting back to keeping hamsters now that we have internet access to info, was the mistaken belief they will just gorge themselves to death, like some fish do. Hamsters won't let themselves starve, nor will they eat till they burst. Once a hamster settles into his life with you, you'll find many times they simply don't care about food and will refuse even a treat on occasion.
Overweight hamsters are usually a result of improper balance of exercise, fat and related contents in the only food they are offered, and similar imbalances in their lifestyle...its not all that usual that a ham will be like people and suffer an eating disorder resulting in obesity simply of their own free will. Hamsters, so far as I know don't do the 'no one loves me, my life is boring, I'm just gonna let myself go, kind of eating ....
Diet, ample opportunity to work off the fat and energy seem to keep this in check most times.
Their weight also fluctuates over days, not weeks and months. They can weigh in at different weights in a very short period of time, so expect a bit of variation in their 'average' weight.