Here’s just one example:
The harvest mouse has the longest recorded penis in all of the animal kingdom. This is relative to the size of the animal, of course.
It is hard to imagine that the cute animal above, at a mere 17 cm long, has an 8 cm penis. The male harvest mouse must have a formidable penis length. Otherwise, he will not be allowed to mate with a female. The female ultimately decides who her partner will be depending on penis size.