I think the large size and aggressive behaviour of mud crabs is very likely Vivec's doing.
As you may know, a few thousand years prior to the time of ESO, Vivec inserted himself into the ancestry of Tamrielic crustacea. The dreugh "Ruddy Man" was one of the monsters created as a result of Vivec's promiscuous couplings with the daedra.
That addition to the crustacean gene pool is evident in the mud crab population of ESO. It is the red, or "ruddy" crabs which are the most directly affected. They are the strongest and most aggressive of the mud crabs. It's almost as if they have inherited from Vivec an instinctive drive to conquer heaven by violence. Sometimes you will even encounter a red mud crab standing, waving it claws around in front of an audience of one or more brown crabs. It is easy to imagine that they preaching, spreading the violent teachings of their unholy god-ancestor among the rest of crabkind...