Can a NEMA 17 with a 10:1 Gearbox REALLY output more torque than a NEMA 23 with a 3:1 Gearbox if the output speed of the gearbox is fixed at 60RPM???
High speed motors have much greater power density than low speed motors, so your results aren't particularly surprising. When building model airplanes we often have to decide between a heavy direct-drive motor and a lighter geared motor.
For model airplanes the heavy direct drive motor typically wins after considering the weight, cost, efficiency, and complexity of a gearbox. But in your situation both motors would require a gearbox so its not surprising that the Nema17 wins. Just make sure the 10:1 gearbox survive the planned load.