Antipsychotic Weight Gain

Out of interest, does anyone know what the exact mechanism is behind antipsychotic weight gain? Is it simply an overactive appetite, or is there some other factor at play? I've been on 20mg Olanzapine for a few years now, and I've gained a good 20kg / 45lbs. I'm a 1.94m / 6'4" male, so I can get away with a bit of weight, but I am concerned about metabolic syndrome and any possibility of developing diabetes. Of course, it doesn't help that Olanzapine is the only antipsychotic that has worked thus far. My psychiatrist isn't concerned about the weight gain at this point, but I thought I'd see if anyone here has some ideas.