Aloicious:
I havent experienced any direct weight gain that i know of on estrogen. but i have experienced some change the ratio of muscle to fat. it'll definitely make you softer. And i think above all, for a gainer/feedee, thats a big part of the goal.
Depends on the person about muscle loss being the goal.
That said, muscle is denser than fat. So, it is possible that you can get fatter without getting heavier.