The hat is too small. I have a fat head, too - getting measured for mortarboards for graduation with my classmates was a revelation - and most off-the-shelf hats look a little weird on me and give me a headache from being too snug.
I assume he has enough money from grifting to be able to afford custom hats. Does he still party like he thinks he's Hunter S? Maybe he hasn't invested in hats that fit his head because he's still constantly losing the cheap ones.