I bought the XL size for my Osprey Aether 85 after reading on a discussion forum how huge it was. (The user had actually used it one night as a makeshift bivy.) I liked the idea of a cover with lots of room to spare which could be cinched down, leaving no uncovered spots, and which could accommodate other bulky items I might want to hitch to my pack. Well this one does all that with still more room to spare. It is enormous.
Within reason, I don't see how a pack cover can really be too big -- as long as you can cinch it down. Better to have room left over that you don't use than to not have it when you need it, I think. On a recent week-long trip I used this many times to cover most of the stuff I wasn't using, much of it just tossed in a heap on top of my backpack. It drizzled much of one day and all one night and everything stayed dry. Perhaps the reviewer who reports water getting through encountered much heavier rain. But so far so good in my experience. Time will tell...