I took an antipsychotic several years ago (quetiapine, aka seroquel), and while I had some pretty nasty side-effects, I didn't gain any weight while on the medication. And I was on it for a year. I've actually only had intense weight gain as a side effect from two meds, and neither were antipsychs - one was an SSRI, the other an SNRI.

So, no. Everyone reacts to meds differently. One med might make someone gain weight, while another person might actually lose weight because of the same medication. It depends on how your body reacts.