I would imagine it's because
A) Lower metabolism
B) Liver's working overdrive on HRT
That's why we're supposed to limit toxins.
I never had much muscle mass, but if you used to be pretty beefy, that would also do it - most people report losing a lot of weight MTF from muscle mass.
I've always been a cheap date, I'm just moreso now. I used to not drive, so I always had a DD. Now that I have to drive myself home at the end of the night, I've cut back a lot. As well, I have not had a name change yet, so I do anything I can to avoid being carded. It also got to be that at house parties, I'd end up getting tired and passing out (Not blackout drunk, but liquor tends to make me tired. It's a depressant after all.) By cutting back, I have more energy.
Also, women need to be much more careful about alcohol consumption, anyway. There seems to be a correlation between me drinking and my top coming off. But then, the parties I go to would make most of the "Oh I'm asexual, I can't possibly conceive of ever being with anyone, even post-op" crowd blanch.