The majority of large fighters I have watched will attack a smaller fighter in an initial flurry and then step back to survey the damage they've done. They tend to get frustrated after a few of these bouts. It is then that the smaller fighter takes the offensive. By NO means am I suggesting that the smaller fighter stand their ground and take the attack directly on their defense, but a well built defense will certainly confound your opponent and keep you alive long enough to strike back at your opponent. Being able to move quickly is a key part of stance.
Movement keep our opponents from targeting gaps as easily and opens gaps in their defense.
Not to mention it tires them out. To stay as mobile as possible keep your weight evenly distributed on the balls of your feet. Try not to put too much weight on the front foot as this will effectively keep you planted for that critical moment as you shift weight to move. Too much weight on the back foot allows you to be easily pushed about and keeps you off balance. This latter problem is the one most commonly encountered by slight and smaller fighters since we tend to fight while moving backwards.