Well, the general idea is that HRT slows your metabolism to what a female normally has or close to it. However, when people start HRT they tend to lose muscle mass which while further lowering metabolism, also has the effect of dropping pounds since muscle weighs more than fat.
I have been losing some weight too, and I am a bit fearful of some of the effects of lowered metabolism, but luckily we have a fair degree of control with exercise, on how our metabolism is. While I am not on HRT yet (still waiting for an interview, then they have to do labwork, then they will work out a regimen so it could be a few months) I have been eating small meals spaced throughout the day, and exercising. This has noticeably increased my metabolic rate.
I went from about 230lbs a few years ago, to my current of about 175, but my goal is still 140, so I am still packing a few pounds more than I should (which at the rate of .5-1.5 a week that I have been averaging I probably won't reach before HRT) So I will probably have to deal with a bit of loss while on HRT. I will be sure to let everyone know how that goes.