How a forehead gets fat:

The body usually deposits fat in certain areas first. Once those areas start to get really fat, the body will start to deposit in other areas which don't have as much fat. In the case of these fat people, it is their forehead. They have sooooo much fat everywhere else that the body just decided hey, the forehead is thin, we have room to lay down fat there. Let's do that since everything else is overflowing with fat.