Why are rat testicles so big?

The first sight of a male rat can be a shock to the uninitiated. Those testicles, giant pillow-like things hanging behind the rump, seem to take up one's entire field of vision. Rat testicles -- also known as torpedoes and goolies -- give rise to a lot of humor and embarrassment.

So, why are rat testicles so large?

It has to do with massive amounts of sperm production. Read all about it here.