Originally Posted by cyccommute

Close but no cigar. The first production mountain bike geometry...the Specialized Stumpjumper...was closer to the Schwinn Excelsiors on which the Marin County mountain bikes were based. Touring bikes and touring bike geometry was a solid road bike geometry with slightly longer wheelbases and slightly slacker head angles than racing bicycles. In fact, cyclocross bikes were based off of touring bikes that racers used for winter training and touring bikes have little in common, frame-wise, with mountain bikes.

There's also more to the changes in mountain bike frames than just lengthening the top tube. The rear stays have been made shorter to tuck the wheel under the rider so that the wheel doesn't spin out as much, making climbing easier. The front to center of the bike has been made longer to balance the rider between the wheels so that the front wheel doesn't come off the ground while climbing and the rear wheel stays engaged. Head angles have been steepened to reduce wheel flop...a big problem with a 69 degree head angle of the Excelsior. The changes aren't about keeping the rider from bashing their knees but about making the bike a better hill climbing machine.

You are right about the tire width. That's just a small part of the differences and certainly not the primary one.