It is best to fight a big guy if he is the initiator/agressor in the fight. If he is the initiator/agressor, make him come to you. If you stay just outside his reach, and he is the one who wants to fight, he will keep coming at you. Keep moving, using your speed and endurance. It would probably be best to stick with jabs and backfists for hand movements and quick roundhouse kicks and side kicks to the knee and shin. The object of the punches/kicks isn't to knock him out, rather it is to chip away at him, further slowing him down, while taking away his will to pursue the fight any more.
Also, the advice on lateral movement is a good one. simply moving back probably won't suffice, because even though you are faster than him, your probably not faster than him running away straight backward if he his runing towards you straight forward. I like the idea of moving in a sort of arc motion. Imagine a pendulum moving back and forth, only after every arc, the rope gets a little longer. This will force him to move both forward and laterally. Also, don't move laterally in only one direction (going only clockwise or counter clockwise) it is much less taxing on him. if you switch directions he has to go against the momentum he has generated to go in the oposite direction. So will you, but since you are lighter and have more endurance, it will be less taxing on you than him.
One thing to watch out for is the charge. If he is getting frustrated/tired, big guys will often just charge you straight on in the hopes of overwhelming you. The best technique for this is strong lateral movement. If you move fast enough and far enough to the side, his momentumn will carry him past you and you will find yourself somewhat behind him. From here, you could take the opportunity to attack him from behind in the knees, legs and lower back (don't bother hitting his upper back/lats If he is muscled), or you could use this opportunity to get more distance/run away. I recomend the latter.
As to why I am letting the cat out of the bag on how to beat big guys if I'm one of them, notice that almost none of what I have said will work if the big guy is not the agressor/initiator/attacker. Since I never have been, nor ever plan on being the agressor/initiator/attacker I don't mind helping people learn how to overcome larger/stronger opponents who are preying uppon them.
Jon