Size doesn't really matter in dead fight. Of coz, in Olympic you will avoid attacking dengerous spot. In streetfight, bigger means bigger target and depends on the surroundings as well.
That's almost completely wrong. Size gives you more range, which is what usually decides fights. In unarmed fights, or fights with short weapons, it's pretty much impossible to defeat somebody with even just 10cm range advantage if they have at least basic martial arts training. Being able to throw a clean straight is enough to attack you from a distance where you're unable to attack them properly, yet still to close to dodge, even if you're both faster and more experienced. I regularly have that situation in sparring - people who I can defeat in one or two strikes with knives or swords, yet can stop me from reaching them for hours unarmed, just because their arms are a bit longer.
The only exception is when you're able to (effectively) attack the limb used to attack you, but that's mostly limited to bladed weapons and FMA-style fist destructions (which require a high amount of conditioning most fighters do not have).
All the typical methods taught in martial arts or self-defense courses for dealing with larger opponents, like moving around, going in low etc. only work if the enemy makes a major mistake.