Yes, you hit on the biggest reason why people gain weight when taking antidepressants. Being nonactive. When you are depressed and lay in bed or are not motivated to exercise or continue in regular activities of course your going to gain weight. Good point.....
I was on Zoloft for a couple of years, and gained quite a bit of weight too. I have a theory that the Zoloft created some sort of "super fat" that is immune to diet and exercise! (LOL)
Hang in there!! It (the weight) will come off with work, good nutrition, and time. Take especially good care of your nutrition now that you are off the anti depressants, it will help you with your positive mental health.
Best to you!