Many mtf are fat because they were to begin with as men. Many men who are fat have low self-esteem and dont have many friends or a relationship, and can generally feel that they don't fit in with society. They compare themselves to other men, who they see as successful and "manly" and conclude that being a proper man is just too hard or not worth it, so they become women hoping it will change their fortunes with friendship, love, and acceptance.
The first irony is that the masculine features these men exhibit, features that would aid them in attracting a mate if they dropped some weight and maybe had an attitude change, become a detriment to the trans female, making them what's known as a troon. The second irony is that losing weight and becoming more sociable as a man is in fact a much easier way to attract a mate/gain acceptance than trooning out is. Should a troon hypothetically lose a bunch of weight and learn some social skills you'd have a thinner more sociable person, but but one who ruined their appearance with hormones. Whereas with a man you'd have an attractive and social individual.