I'd agree that it is possible to put on muscle mass after 4 months of consistent cycling. The other thing that happens and also results in some puffiness (besides inflammation, someone else already mentioned this) is that you are storing more glycogen in the muscle. For every gram of glycogen stored in muscle you must also store 3 grams of water. This settles back out again with time. It also goes hand in hand with general hydration which Grog was talking about.