For that application for me the medium is plenty. I put food, Steripen, headlamp and gps in mine. The large will fit a pair of 15x binoculars. A medium also fits a beanie, couple pairs of lighter gloves and schmag in it easy enough.
Second on the medium! I run a medium on mine because i know if I put a large up there i will put too much crap in there and it will get kinda floppy. A large would kinda look out of place imo. When I run a guide lid with it, I take it off the collar and put it on the lid. You could easily run both, but I havent need that much space yet.