I found an interesting article about olanzapine here. It explains why people get fat on it - because it makes the body use fats as a source of energy, instead of carbs, by keeping the insulin in the blood gigh all the time. So no carbs are used for energy (well, little), instead they pile up in the body. Also, it increases body fat, but at the same time decreases lean body mass, so you get fatter.
Basically - the insulin is always high, so the body thinks it didn't get enough carbs. That's why you get carb cravings. Also, because body thinks it doesn't have enough carbs, it starts using fat for energy. But the carbs are there, and if they're not being used for energy, they get stored somewhere. Also, because it's always hungry, the body starts to store more fat (as it happens when you're on a diet, and you're always hungry, instead of burning fats the body starts storing them, because of the "famine").
Interesting.