Because they like to party. Get it? They have parties. Ha, ha.
Seriously, many male animals have inordinately big testicles. If you think Hamsters are large, you should see those on some of the monkeys of Asia. Wow !!
The testicles are large because of the high sperm production. It's high, because they breed a lot to insure the survival of the species. Hamsters, like all rodents, breed very, very often, and reproduce like crazy.
Professional Wildlife Cinematographer, Photographer, & Naturalist 38 years.