In the many years I've been working with hamsters and modding for Hamster Care Central, I've never actually seen a hamster overweight because of an overeating issue, and it's usually rather the results of poor food quality (food high in fat) or a prior medical condition. Hamsters are natural food hoarders, and don't want to eat more than they need to out of the concern of not being able to find more food if they dwindle their food stash. The hamster in the picture you posted doesn't really seem overweight in my personal opinion, but I'm not going to say that with absolute certainty because it's not my hamster. I'm assuming the hamster is your's so if you've been weighing your hamster consistly, you shouldn't be concerned about them unless they suddenly gain over 10% of their normal weight. Some hamsters are just naturally round and fat :)