I have never been on risperadol, but I have, in the past, been on prozac(I can't remember the dosage) and didn't experience weight gain.
Perhaps, it's the risperadol?
Check with your doc. Also, have you had a thyroid function screening(lab bloodwork) lately? It might be something you may want to look into, while going through this. I know how frustrating this is, from my own experience. I wish you lots of luck, in getting to the bottom of this.
Just my 2 cents.

DE