Right whales have the biggest testicles, weighing up to a ton. They have one of the tiniest brain to body mass ratios.
Right whales don't have teeth or horns or claws to prevent the other males from mating with the female, so they have developed an interesting way to compete called sperm competition.
Each male produces massive amounts of sperm in an attempt to dilute other sperm that might already be in the female.
To do this, the whales need the appropriate plumbing, which explains why one southern right whale testicle weighs in at over 500 kg (1,100 lbs), making the right whale the animal with the biggest testicles.
At 1% of the whale's total body weight, this size is very large even taking into account the size of the whale.