There is another reason for the difference in numbers that hasn't been mentioned yet in this thread. For many MTFs crossdressing is primarily an erotic activity (or at least it started that way). That doesn't seem to be the case with the FTMs.
Now I'm not saying that there are no erotic aspects to the FTM experience. Far from it! Just that they are doing this to express their gender identity, whether they be TS or not. Whereas there are plenty of MTFs that identify as males and don't ever think of themselves as females.
I guess what I'm saying is that there are plenty of FTMs that identify as TG, but very few who would think of themselves as fetishistic transvestites.
I'd love to hear what the FTM blokes have to say about these ideas.
Robin