Did you know the animal with the biggest testicles relative to body size is the Hamster... 
Maybe for a mammal but you said "animal". Don't short the mighty katydid (bush cricket
)!...
"The reproductive behavior of bush crickets has been studied in great depth. Studies conducted in 2010 at the University of Derby by Karim Vahed, Darren Parker and James Gilbert found that the Tuberous Bushcricket (Platycleis affinis) has the largest testes in proportion to body mass of any animal recorded. They account for 14% of the insect's body mass and are thought to enable a fast re-mating rate."
