My son was taking a very low dose of Risperdal for a short time when he was younger. It seems notorious for weight gain. He eventually lost the weight on his own but it did take some time.

I have read and also heard from others about weight loss while on Abilify (Aripiprazole) in the past. I've also heard of it being weight neutral as well, so I don't know. Hopefully someone will come along who has experience with taking it.

You might also want to check in PubMed .
There are a number of articles in there relating to "weight loss and Abilify". Some are comparison studies... e.g. this one below comparing weight loss between both olanzapine and aripiprazole.
Comparison weight change - olanzapine or aripiprazole