I seriously doubt that ftm guys willing to wear dresses and heels represent a sizable portion of the population.
When I call myself "feminine," I mean to the extent that I like feathers and rainbows (though I dress in jeans and t-shirts or button ups), I giggle, I am totally cool with chickening out of certain risky activities, I'm skinny and soft and foolish, and I don't mind talking about feelings or vulnerability.
This is still far far more masculine than seems to be acceptable for a female-assigned person in our society. And yeah, ID'ing as male is pretty much the main thing it comes down to. I'm a guy.
I am tough and dirty and clever. I'm not sure what else. I'm still learning which qualities I'm supposed to have for people to accept me as male, and I kinda refuse to strive for the ones that don't suit me.