There have been studies were they've given guys hormones and just let them veg on the couch so to speak, many gain 8-12 pounds of LBM give or take a pound or two.
The question of ex-lifters that don't train, yet gear up, is interesting. They, too, probably wind up in the same boat as the above, it's just slower and less apparent to the eye because they're coming down from such a high starting point. We've all seen the deflated mass-monsters here:).
Now, probably the best way to handle it, is just to cruise at something like 2 workouts a week. I've seen plenty of guys maintain 80-90% of their build doing that.
