I'm on citalopram and have been on Fluoxetine in the past and have had no problems with losing weight, however each carry side effects of weight gain or weight loss, so obviously affect people differently. It may be worth however having a chat with your SW consultant and/or your GP if you have any worries.
Just a add, although I don't suffer from Bi-Polar disorder, I do suffer with severe depression and have found SW has such a positive effect on me. Don't know whether its cos i'm eating fresher, healthier food, or whether its the exercise i'm doing or whether its cos i'm enjoying how I look now, possibly a combination of the 3 really. But I feel like a different person. I know my depression will never 100% go away, but the diet has given me what seems like 6 months of respite from the constant mental beatings I was giving myself because of simply being me, which is worth every penny!
Good luck xx