Hi E.B.,
If we're looking for a cause-and-effect explanation for why we get fat, regardless of surgical status, I believe we need look no further than the amount and type of food we habitually eat versus the calories we habitually burn during the course of our daily activities. A change in our hormone (gonad) status can and often does alter our metabolism, either speeding it up or slowing it down. (Sadly, slowing down seems more common than speeding up; I guess it's partially an age thing.) What this means to us on a practical level is that, if SRS alters our hormone status, which slows down our metabolism, then we will absolutely gain weight unless we compensate for the slower metabolism. That is to say, we must increase our activity level (via exercise for example), and/or we must decrease the amount of food we eat (especially rich, sugary, fatty foods).
As for me, I am more lazy and sedentary now than I ever was in my youth. I do, however, pay attention to what I eat and how much I eat. I deny myself no culinary indulgence, but I do moderate how much and how often I eat. Consequently I am 25 to 30 pounds lighter than I ever was as an adult pre-surgery. (At 5 foot 7 inches, I deliberately maintain a steady weight of 142 pounds.)
In my experience, the key is to make up my mind what weight I want to be, and then control the things that are within my control (how much I eat, how much sugar and fat I consume, and how active I am.) In this way I never, ever feel that weight gain, nor weight loss, is out of my control.
I hope this helps.
Love, Miharu