We have two clownfish that we bought together however they were not sold as a mated pair. As far as size, it is hard to tell as they look darn near identical in size.
One indicator of a potential health problem is that he is holding his mouth open. I just observed again and our other clown opens and closes it's mouth but the one in question is swimming with it's mouth open. I couldn't see any signs of a growth or anything in or around his mouth. The clown is also holding his pelvic fins closer to his body that normal.
When I talked to my LFS
yesterday and described the behavior they said the same thing - 'welcome to the world of the birds and the bees' - however, I still think there may be a potential health issue ...???