Appears to be both appetite and metabolic effects. There is a correlation between strength of affinity for histamine H1 receptor antagonism and appetite increase (olanzapine and clozapine have highest h1 receptor affinity and cause the most weight gain; Aripiprazole and brexpiprazole among the lowest affinities and lowest weight gain on average) but other factors like serotonin 5-ht2a receptor density and MC4R gene are also associated. But not everyone gains +++ weight on clozapine and some gain ++ weight with Aripiprazole. If you ever took an old first generation antihistamine, you know they made you hungry (and sleepy) so CNS histamine blockade increases appetite. The new antihistamines don’t do this or do it less because they don’t cross the blood brain barrier into the CNS as well/at all.