I think since the other heads are all normal-sized, the point is that his head is too small for them to want it as a trophy

I figured the joke was that they're fattening him up.

I kind of want to settle on this read, and the thing that jumped out at me was also his weirdly small head. The trophy skulls everywhere look regular size, though. If this is a head-shrinking bit, that seems incongruous.

I don't have other ideas though, beyond "fat man on his way to get eaten"... which I guess is maybe enough. Sometimes they're that simple. His head looks so weird though, I want it to mean something.

Edit: I think Lux has it. He's shrunken his own head to make it an undesirable trophy.