Without seeing the 75mm motor it's hard to say what they may have been thinking. Most motors, brushless included, are mostly limited in continuous operation by the heat generated by the armature windings and the wear on the bearings. The armature dissipation limits current, while the bearings limit speed. In some applications you can significantly reduce the size of the motor by providing adequate airflow and, in the case of conventional brushless motors, heat sinking to the case.
Just for chuckles, consider a model airplane motor that's about 20 times smaller, yet is rated for up to 55 watts (but not at 4000 RPM).
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motor-2200kv-eflm1130
It works because it spins faster, because it's hanging out in a strong airstream, because it's for intermittent use, because pilots are willing to do maintenance on the airplane far more often per hour of running time than a copier owner would, etc..
So with adequate cooling and a scheduled maintenance program, you can get quite a bit smaller than the motor you found.
15W is not a lot of power. Check out Pittman 4444 series, they are NEMA 17 size, I think, so 43 mm dia. Actually, seems they are smaller than that, I'd think closer to 30mm. Depending on the stack length, you can certainly get 15 W out of one.
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