That's an interesting concept, VashtaNerada, and I wouldn't be surprised if how society dicotomises gender somehow causes transgender people to want to associate with another body type, because it's not socially acceptable to want to dress and behave like in the box where women are placed (or men are placed).
I suppose it's ok(ish) if how you want to be seen is somewhere between the sexes, but if you are, say, male, but your preferences match more with the extreme socially associated with extreme femininity, then maybe some people can only reconcile it with themselves to actually become fully female, in that it would make more sense to them. Would they still have that problem if, say, it was perfectly normal for men's appearance to be from short hair, plain clothes, sensible shoes to long hair, no beard and make up, dainty shoes? And the reverse for women?
On the other hand, because there is evolutionary pressure for males to make sure that they have fathered their children, it is likely that it evolved into the clear sexual dimorphism that we see today, both at innate body level (more hair, for example), but particularly in terms of appearance (e.g. cutting hair, wearing trousers at some point, and so on). I know at some point it was a crime to dress like the opposite sex, because then you (well, any male) can tell who is a potential sex partner or a threat. The same worked and still works for social and professional classes in a way.