Ehmn, it does happen to lose weight on HRT. There are two things conflicting when you get on HRT: your metabolism decreases, and your appetite decreases. Whether you lose or gain weight is all up to which is stronger... Which, unfortunately, for most people, means weight gain rather than loss. Since, well, appetite is partly physycal, partly mental, and HRT shouldn't change much how a person likes to eat.
For me, though, HRT has, eh, stabilised my weight. I had been losing some due to my ADD meds reducing my hunger to mostly manageable levels (hallelujah), but when my metabolism diminished, I guess it compensated. But maybe it's even unrelated. The period for which my weight was stabilised corresponds with HRT, yes, but also with the start of a busy school term during which I didn't eat very well since I stopped cooking, and during which I also stopped doing exercise.