Should you be a little specific it would have been easier. The antipsychotics and antidepressants by themselves are wide classes, and they have mechanistic variations. Although its still possible to make a rough guess it's much better if you make it specific. In which class are you particularly interested ?
As Mohammed said, this is difficult to answer without specifics due to the broad membership to these medication classes. What I can tell you is that many antidepressants and antipsychotics have inherent antihistaminic activity. This is one of the mechanisms by which increased hunger and weight gain can occur (eg mirtazapine).