I'm guessing it is water retention and increased glycogen storage. Any steroid will cause increases in water volume and glycogen within the muscle. A similar thing happened to me - I just finished a 16 week cycle. I started off doing test e 500/wk, and DMZ+M-lmg (Phorce). I put on 20 pounds in about 3 weeks time, much of it water and fat. I went from 190 - 210lb, and although I looked beefier, it wasn't the aesthetic I'm looking for. I prefer lean and tight over big and bulky. So I went off orals for three weeks, dropped my calories significantly, and started losing weight. I decided to end my test cycle with an epi bridge, and did the last 5 weeks on test using epi at 40mg/day. As soon as I started the epi, my weight loss stopped (on the same calorie intake), and I actually gained back 5 more pounds. I ended my cycle at 210lbs. I am now in week three of PCT, and I weigh 200lbs. My bf % has stayed exactly the same - so I basically shed 10lbs of water, even though I was using a dry compound. Granted, I was on Test E still, but I think you get the idea. A lot of the rapid weight gain on cycle is water and glycogen. Tissue takes a long time to develop, and if you've ever held a pound of raw chuck in your hands, you know thats a lot of tissue. If you gain any amount of weight on cycle, expect to keep about half of it, cuz the other half is H20 and sugar!
