It is impossible to visualy sex these fish unless they are spawning. There are no fin differences and the body differences only show up when the female is particularly gravid. The size difference is slight and you need to wait until they reach full size to judge this. Considering they get to at least 10" (usualy more), it's unlikely that yours are fully grown.
The size differences you've mentioned are almost certainly because you have only two. As a result, one is dominant over the other and therefore slightly hinders the other's development by out-competing it when it comes to food and forcing it to use up energy to combat stress.