He goes through changes based on mood. His head has been bigger before. it was about the same size a few days ago. When the RTM bred, they did so next to his territory. As a result of their aggression his head went down. Yesterday he set up shop on the other side of the tank. I think the loss and gain was triggered by territory.
His head blew up in about a day. It can get bigger. I figure he'll look a little top heavy by the time he hit's 8" i measured him yesterday (through the glass) and he's already at 6".
BTW thanks for all the comments. The cat that was commented on is a type of Syno. common name is Domino cat.