i saw an analysis of this on the outdoor network once
grizzly has much denser skull, more powerful "arms" and longer claws
also is much meaner on average, living in greater competition of numbers for survival, so the "on" button is more quickly depressed and, more to the point, it has more "hand to hand" combat experience
back in the California pioneer days, they used to have pit fights for entertainment and gambling fun. started out grizzlies vs. bulls. this soon had stale betting, as the grizzlies became adept at quickly dispatching even the largest bulls, often with a single swipe that crushed the bulls skull. so they brought in African lions to liven up the betting. same result, the bear pretty much always won and fairly often crushed the lion's skull with a single blow.
the show on the discovery channel concluded that most likely the grizzly would wind up crushing the polar bears much thinner skull, btw.
also, male grizzlies can go over 1K, and if you consider kodiaks that size is fairly common. the largest bear ever killed was a browny, not a polar, btw, a kodiak that was around 2500 lbs (largest polar bear ever around 2200 lbs).