So I have a ballscrew and a stepper motor that would like to interact with each other with a 2:1 gear ratio. I thought to myself, no problem! My shaft diameters are 10mm and 3/8". My stepper motor is rated for 320oz-in at about 300 RPM.

The largest pulley can have a total OD of 2.125". Cool! Great!

So I look around and lo and behold, I can't find anything larger than a 3/8" wide XL pulley that will fit a 10mm or 3/8" shaft. Come to find out, according to SDP/SI whitepapers, a trapezoidal timing belt in this size is really only good for... 90 oz-in with a 16 tooth minor pulley. What the fart? Alright, let's try a 5mm pitch x 9mm width HTD. Great! Oh no. 280 oz-in. Damn.

I can size up the belts but then I need a shaft larger than 10mm or 3/8".

Am I missing something?