What is more important in a fight, 100+lbs of weight or skill?

[kurosh]

I'm having an argument with someone and it's really pissing me off. They claim a 100lb 15 year old boy could kill a 300lb 6'7" muscular man, if the boy has been trained extensively in martial arts and the man has not. They also said some bs about seeing a 60 year old 100lb woman doing the same thing. Can anyone knowledgeable about this chime in? I wrestled for a while and there even 5lbs made a huge difference. I don't care if it's a little Bruce Lee, 200lbs of strength and speed is too much to make up.