Male humpbacks are really a sight to behold in the mating season. Often a female will reject their advances and roll onto her back at the surface, with her vent in the air. A really large male can still mate with her though, by getting alongside and rolling onto his own back, then sending his weirdly mobile penis off to explore by itself.
Interesting, since that would provide strong selective pressure for greater size.
(Or were you not referring to body size?...) -- Mike Keesey