I can't speak to the stacking for size M, but it might be possible that you can fit in a small. I'm 5'9, 150 with a 30 waist (measured circumference 32 in) and 30 inseam, and I need a belt to wear my size S P10s at the waist. FWIW, I have both the p10-s and p10a-e, and the p10a-e's fit relatively looser on the waist for me.