Combining weight gain with a male to female gender change is sometimes called "Male to BBW" and it's a topic I enjoy. I wrote one book which focuses exclusively on Male to BBW, Trisha's Girl. Also, The Transformation Mall has a chapter which features it. Still, most gender change stories - both my stories and those by others - don't include weight gain (and, of course, most weight gain stories don't include gender change). The question is: why?
Part of the reason is that oftentimes, a gender change story causes a man to turn into a woman rich in what I'll call "traditional beauty." That means she's usually young, feminine, with a thin frame and large breasts, exactly what our culture traditionally sees as beautiful. Weight gain would add a complication to this since it isn't generally considered attractive through a mainstream lens.
This raises a further question: why do many gender change stories prefer to steer things towards traditional beauty? Partly, it's because some of the appeal of gender change is to embody feminine allure, to appear pretty and sexy to the general public. Another reason may be that some men who fantasize about gender change simply want to turn into the kind of women that they personally find attractive. Finally, transforming into a woman often also involves taking on a submissive sexual role, and being small, thin and frail is in some ways lends itself to submissiveness more than being fat (or being muscular.)
Stories which employ male to BBW can buck these trends in different ways. For example, in Trisha's Girl, the weight gain is described as unequivocally sexy, and helps jump-start the male character's porn career. By contrast, in "The Black Rose" shop in The Transformation Mall uses the added weight to further humiliate the misogynist man who transforms into a girl, though later on he too finds acceptance and love.
What about you? Do you prefer weight gain and gender change together, or separate? Why?