I'm on 20g myself and have lost weight.. slowly but its a loss.
(40lbs in 8 months)
I talked to many pharmasists and also psyciatrists and I found out that prozac doesn't make you gain in low doses like 20g and zolof and others have more the tendency to make you gain.
Now, You are smaller than me, maybe there is a ratio of meds to size but I'm not sure.