I always think I would rather be slim and unwell, until I actually am unwell and realise being suicidal is quite shit.
It is possible, usually, to find a decent combination of meds which won't make one extraordinarily fat. It's a hard slog though.
Contributing to a lot of the problem, I think anyway, is that there are so often comorbidities between 'other' mental illnesses, and eating disorders. Disordered eating behaviours can be part of the symptoms of other illnesses, like for me, I know I am manic when I don't have to eat for six months and lose a shit ton of weight. Equally I know I'm depressed when I start bingeing and feeling deprived.
Bleugh. It's complicated, and hard.