First, understand that I'm no mechanical engineer and am not qualified to speak to the issue of "limits" regarding any mechanical device. Nevertheless, it seems to me that the horse power of the motor, aside from having the capability of providing greater torque under load, shouldn't make that much difference. A 1750 RPM motor will run at 1750 RPM (that's usually a no load rating) whether it's a 1/3, 1/2, or 3/4 horse power device. The 1/3 HP will stall with less load than the bigger motor but I don't see what other difference there could be.
050207
Yes, I suppose that the higher torque could cause the pulley to come apart if things got jammed up; like a BIG catch ... But this is a belt driven machine and I'd expect to see belt slippage before pulley disintegration, especially on a keyed shaft. There is one aspect that I overlooked, that being the difference in shaft size from 1/2 to 3/4 HP. The driving pulley from the 1/2 HP motor is probably designed for a smaller shaft than the 3/4. I'm interested in the technical info. you find in your research so please share what you learn.