The best way to see if it will work for you is to load it up and put it on. Walk around with it on and try to note what feels good, what doesn't. Then go down to your local outdoor store, assuming you have one, try on different packs with a load in them and try to make comparisons to the one you have. If they have that Osprey model in a medium, all the better, load it up and put it on for a while.
Just because your pack is a large and you feel like you're a medium, doesn't mean it won't work for you. The fit of every pack is unique, the more you try, the better your chances will be of finding the perfect one for you.
Good luck!