That does sound like a rather cruel thing to say!
I have been off and on all sorts of meds for my bipolar disorder. One drug (Remeron) made me go from about a size 12 to a size 18 over the course of a summer! And then it's not that easy to get the weight off, alas.
At one point I mentioned to the pdoc I had at the time that I didn't like my weight. And he said, "Well, I'm trying to balance out the drugs that cause weight gain with the ones that don't!" That said to me, be happy you're stable, anyway!!
I have cut down on my calorie consumption. I take Wellbutrin. And I'm not on all the meds that pdoc had me on. I'm also trying to walk some, even though I have bad foot pain that I've been getting treatment for. I have lost some weight, but I guess at my age (late 50's), I don't expect to be a svelte person again. My identical twin sister weighs about 35 pounds less than I do, but she has had Stage 4 cancer and a problem with digesting fats. (And I don't want to have those experiences!)
At any rate, I say, "Give that weight loss your best shot!"