Seems most models end up gaining some weight. In some instances it is ideal as they started off rather thing to begin with (e.g. Jalyn - although it's rather subtle, but still some weight gain), and then there are instances were it is a little too much for some (e.g. Felicity - I'm fine with it, but some don't care for it).
I've noticed that since RachelSinger has moved to the UK she has gotten a really unflattering dye/streak-job in her hair, braces and she is almost on the verge of being anorexic. She was thin to begin with, but never THAT thin.
AwesomeKate has fluctuated up and down. I actually prefer her with the additional weight as it seems to go in places that emphasize her curves.
I've noticed that alcohol consumption (lots of empty calories) or the amount of income the models bring in seem to contribute to the certain models who go up as they usually have a steady and ample income from camming. The models who are on the very thin side usually either just drink and don't eat much or they are on the lower end of the pay scale. And then you have girls like Brina_ who posts all sorts of junk food porn on Twitter, drinks energy drinks/booze and never seems to gain an ounce. She is has an amazing metabolism, workouts or just fucks like a jackrabbit all day long to burn it all off, heh.
