People who are driven to the delusion of transgenderism are trying to live out an escapist fantasy, much like people who fall for "body positivity". It's the intersectionality of people who are more willing to indulge in fantasy than diet/therapy, hence why you find so many fat trannies in other weirdo subgroups like flurries, incels or weebs.
Alternatively, most online trannies live in America/Canada/UK and the tranny obesity rate probably falls in line with that of the general population.
Perhaps it's a bit of column A and column B?