Hi, Done;
I've no experience with Zyprexa, but, if it's an SSRI, they are pretty much known for weight gain.
Welbutrin is an anti-depressant that doesn't cause weight gain. But, it can cause anxiety. With me, it caused panic attacks.
I wish you luck. Tell your doc up front that you won't take anything known to cause weight gain. It will eliminate alot of going back and forth; expense; and frustration on both your parts.
Shalom!