i think it might be important to note that different people have a little different reactions to the different medications..
i didn't gain anyweight on the seroquel, but the zyprexa caused me to gain 60lbs :/

i was on 600mg seroquel for a little while... 400mg for about a year or so.. started at 300mg...

i've never used any maoi ..

i hate zyprexa :/ it didnt do anything but make me gain weight
but thats just my experience, it helps some people a lot
i was on 20mg for a year or 2 (its really hard for me to keep up with time..)

edit:
here is an interesting article about weight and APs.. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3998960/
its just an article though, i think basically any ap can cause weight gain.. just some have more reputation for it