Doses for antidepressants depend on the clinical trials of the drug and how well you metabolize it, not your weight. Often, these drugs are started on a taper up to see how you do on them before you get to the recommended dosage just as they are tapered off of to avoid withdrawals. I also hope you're doing this with a psychiatrist and not a regular doc and that you're seeking therapy so you can fix what's wrong and not just take drugs forever. By the way, your Zoloft dose wasn't very high, either.