My guess is any small-motor that develops higher pressure is a "fixed displacement" type motor.
If we were talking a fluid pump, I guess it could be a gear-motor, but those might be a little expensive. Especially I think if it's an air fluid those would not be so small or cheap because air has more leaking.
So instead I guess it's a piston pump maybe? I bought one of ebay with a plastic section on it and it's roughly the 28D size.
Anyway biggest downside to the small high pressure fixed displacement pumps is they're trickier to run high volume. So you need to make your lines small and the total volume low.
(scattered various cents, maybe none useful.)