Why don't you see a lot of very long thin, high revving motors for higher power with smaller size than large motors? This one has completely the opposite scale of dimensions even.
So, mainly two reasons that I am aware of. First, with a long spindly motor running at high speeds, eventually shaft whip becomes an issue; the distance between the bearings ends up being really far. Second, designers usually want low-speed torque more than super-high-speed power, and at high diameters you get a bit more torque than small long motors of equal volume, because torque scales more like the volume of a somewhat hollow cylinder rather than a fully solid cylinder.
For most brushless motors--though not axial flux motors like the one under consideration here--it's much easier to get heat out by passing air axially across the stator. This means that while the surface area is increasing proportionally with length, your ability to flow air across that surface area is not keeping up. For that same motor volume, a shorter and fatter motor will have better airflow and cooling than a long skinny motor.
