Ogryns just Are stronger and tougher than marines. That's why they carry heavier guns, bigger melee weapons, and wear armor that offers similar levels of protection against attacks despite being literally just massive slabs of metal. Space marines are better not because they are more physically powerful, but because they're augmented to be more versatile and better soldiers. It'd be like comparing a human soldier to a gorilla. In a straight-up fist fight, the gorilla is probably going to win... but a soldier wouldn't be dumb enough to let those be the terms he engages a gorilla under, and I definitely wouldn't want a pack of gorillas going in to do the kind of missions I'd give to soldiers.
Physically, the only thing more impressive about a marine than an ogryn is their ability to survive in the long run. That's it.