You can defeat a much larger opponent if you have an ample amount of training, experience, and athleticism than they do. But other than those qualities, the fight is very much in their favor. If you have never fought anyone who seriously means to hurt you, than I don't think you have much of a chance at winning. Performing under stress in a fight is a required quality in any threatening situation.
Here's a few things that may help.
- Kick at their kneecaps - fast and hard, dislocation of them of an opponent who is larger than you means the fight is in your favor, a solid hit will have them collapse from their own weight.
- Light and quick jabs at their throat, it doesn't take much force to shut the esophagus, do not use your knuckles as this will likely kill them by breaking it entirely.
- Bodyshots like the liver/spleen are ideal, imagine a blade in the middle of the fist as you drive up into their torso from the stomach, it's a debilitating hit.
- Striking the back of the head is helpful, often in a grappling or wrestling scenario, if you manage a few hits and they're unconscious, cease immediately as this stands a good chance of killing them too. The idea is here self defense, not murder.
- Clapping the ears with a semi open palm can and will deafen them and disorient them, it's a good chance that you will make them dizzy with repeated hit. You can use your fists for greater damage but I caution against it.
- Headbutting using the frontal ridge of your skull and driving into their facial orifices is also a good tactic, I'd use for someone who is threatening your space and talking shit with intent to hit you, it's a perfect opportunity to pull this off, most people don't expect it.
- A double dropkick can be absolutely devastating when timing is right and the opponent does not expect it, even a much larger opponent will take massive damage if done to the solar plexus or the head, it's a fight ending attack. If you perform this badly or timing is wrong, then you're most likely about to lose the fight in a bad way.