All the above are right on, PPSG smaller, more compact, smaller fork gap, which is the reason new shooters fork hit the crap out of it.
PPMG bigger, a bit heavier, way bigger fork gap, and wider fork tips, the handle its a bit longer also, makin it better for larger hands, I have shot both, and I kept the PPSG, its better fit for me. By the way i shoot 28mm to 22 mm TBG on my PPSG without problems.
Cheers!