Just becuase one guy weighs 200 pounds and is all fat and the other is 200 pounds of pure muscle, it does not automatically mean the "fit" person will go faster.
Your brother probably peddles 10x the rate you do and you probably also suck at shifting anyway....
it depends on wheel size, tires, gear ratios, strength, leg length....probably some more factors too. upgrading your equipment is only one piece of the equation
I have a KHS Flite 500 (road bike)
it's not a mountain bike, but it is 24 speed, and the gear ratios are pretty high. Im on gear 24 basically all the time unless I'm stopped or accelerating
I beat the bus to and from school on it.
that's what it looks like, except the front fork is silver (the original one got bent out of shape and had to be replaced - and they didn't have any blue forks)
well on my mountain bike i used to be able to get up to about 50KM/h on a flat and as fast as i can go without getting scared on the way down hill. that was before the bike got trashed though