Who is more likely to win in a fight?
The skinny guy
Well, my friend, mascilurar isn't a thing... so, seeing as though a skinny guy and martial arts are a thing, I would have to go with him.
Depends on the reach of the 5'0 guy, and the agility of the 6'9 body builder.
Too many variables. What is their age? What martial art does the skinny guy know, fencing? Any rules? Street fight? The first component of a fight is the WILL to fight. Who is motivated to fight? The second component is speed. If one of them is 10x faster, not much else matters. Upper body strength matters. Fighting skills is well down the list.