My hamster has recently gained a lot of weight, could he be sick?

I think he is completely fine :) I got my hamster 1 year ago on my birthday from Pet Land and when I got him he was small and then he became bigger it's just a process of being healthy . Your treating him right and it is great to be drinking a lot of water. He is great so don't worry. But, from my experience just take it to the vet at least once a year. :D