Those are the hamster's testicles, not his butt. Nothing to worry about. Male hamsters have very large ones. I know, because I have one and his are huge, but that's normal! :)
Can A Male Hamster Die If Its Butt Is Swollen?
My male hamster's butt is swollen and has been swollen since I got him 4 months ago but you might still want to go to a vet about this
My friends male hamsters butt is swollen shes had it for 2 months and noticed it 2 days ago hes eating and playing why would this be