I couldn't tell you who made the first, given there would have been experiments with the wheel size from its first inception.
But it is a progression of the discipline/sport/pastime.
Look at brakes or forks or any other part of the bike and development is continual.
As known, the bigger diameter rolls easier over obstacles, so eventually some bright spark would have looked to use a 700, then changed to a 650, thinking about the stresses and maybe the bigger wheel more flimsy.
Not the thing for obstacles :?
Personally i reckon one of the team riders proposed it, as you'd need the connections to be able market it to the entire industry and for them to accept it.