While most women are generally attracted to big, strong men, with seahorses, the best pairings are those where the male and female are about the same size, whether that size is large or small.
With seahorses, it’s the male who gets pregnant–the female inserts eggs into his pouch, where they’re fertilized, and incubated until they hatch. Because larger females produce more eggs, they seek out larger males with larger pouches, which works out well because the males don’t want to waste valuable pouch space that could be used to nurture more offspring. In fact, scientists speculate that this similar- sized mating may have led to the formation of new seahorse species.
New species are normally formed when a single species somehow ends up separated by a geographic barrier, or if some members of the group migrate, and then each group starts evolving separately. But certain seahorse species may be the result of a process known as sympatric speciation.
Basically, this means that in certain environments, smaller seahorse and larger seahorse pairs were more successful than mid-sized seahorses. This way, you get one group of large seahorses, and another group of small ones. This is called disruptive selection. And each group then goes on to evolve separately. You end up with two species in the same geographic area.