My p-doc friend says that if you are bound to gain weight from a medication, you will start gaining weight even at low dosages.

I looked up dose-dependence of the weight gain on Zyprexa a bit and it seems to be not very much dose-dependent - see Weight Gain 'mechanism'? Dose Dependant? - Zyprexa (olanzapine) - The Crazy Meds Forum: Crazy Meds Talk ver 9.0