[WHOOPS! THIS MESSAGE IS EDITED]
Bigger/heavier guys have the tendency to lean on you, to clinch, you'll see this with tiring boxers as well. And depending on your opponent, you should try to lean on him, get some leverage over this guy, or muscle him into a position where you can get some hooks'n uppercuts on him. If you are interested ask your coach for drills relating to clinches, moving your opponent, check out references on the web to, here's a weblink to get the better of a classic clinch. http://www.boxing-manual-boxeo.com/lesson10.html

One sneaky tactic that I like, especially if the guy is disrespecting you, gives you sucker-punch potential in the ring: Get into the position to push before the referee brakes you guys up. Push the guy back and nail him hard in the temple (step forward with a right hook to the temple, and they won't know what hits them) Works best after a few times the referee brakes you guys up. The effect of referee broken up clinches causes your opponent to feel more at ease during a clinch, giving you the chance to catch him with his guard down. This is one of the moves that Jack Dempsey pulled on clinching opponents.

-Rocky (Let the good times Roll!)