I think, in terms of competing, you're probably looking for more strength, as opposed to actual mass--though most guys look for more mass for aesthetic reasons.
I used to row lightweight, which meant I couldn't weigh more than 160lbs. But it took me 4 years after I stopped rowing to get back to the squat poundages I was able to do when I was training. It's all about doing very high intensity (i.e. high loads) at very low reps.
The other thing you have to remember is that your weight class is determined at weigh-in, not the time of competition. The guy you saw who was jacked, might actually be in a higher weight class than you, but manages to lose a couple of pounds of water for the weigh-in. He may also not be carrying much in his legs.
Mass is mass. If you want to put on 10 pounds of muscle, that's going to be 10 pounds no matter how you slice it (unless it's on a different planet). The only thing you need to control is avoiding unnecessary weight in the form of fat. I would time things according to your season. You will likely put on some fat if you're going to make any substantial gains in muscle mass--the going is tremendously slow if you're too obsessed about staying lean through a gain phase. So give yourself time to gain up, and adequate time to lean down, while preserving the muscle gains you've made in your gain phase.
But you can't add, say, 20 pounds of muscle mass without gaining 20 pounds of weight
"In this house, we obey the laws of thermodynamics!"--Homer Simpson