This is a good point. Righty fighting righty would mean the large claws are facing smaller claws. Righty fighting lefty means small claw facing large claw. But in either case both crabs are in the same situation so I would expect no advantage to either. Maybe there are other parts of crab behavior or anatomy which are hardwired for a large right claw? Another question is why left clawed crabs haven't died out entirely if they're disadvantaged?