Smaller bears are not hard to kill, but even small bears can chew and scratch the heck out of a human so they should be approached with caution. Same size as a deer but way more potential to hurt you. BIG bears are tougher to kill. Much more heavily boned than deer, much bigger muscles, more fat to contend with. Bottom line is, you wound a deer and follow him, put him down - he is not likely to chew on your ankle. You wound a bear and sometimes, very rarely, he decides that was not a nice thing for you to have done and he comes for you. Very rare but it happens so why not avoid that and put him on the ground first shot... Use the best bullet for that job and hit him right - no big deal.
Big autumn bears are different critters than big spring bears, their shape even changes. Big fall bears can be at least 20% heavier than in the spring, probably a bunch more - they got butts on them about like Opra's, maybe bigger. When they sit down they look like a big pear - big butt displacement. Belly about drags the ground when they walk. Ears are real tiny, not prominent. No lanky look to them, they swagger.
Big or little, hit the bear correctly and he is yours. All about control, so go for the nervous system initially and the game is in your hands. I like heavier bullets and stout construction solely for the raking or the "precision shots to the spleen" taken as he is going straight away. Nice to destroy the pelvis, blast the spinal column and still penetrate through a foot or two of gut-juice, then the diaghragm and into the lungs. Takes a stout bullet to do all that. When I did bear control I shot a lot of bears with 180 grain bullets ('06 factory loads), mostly going away shots and when I switched to 220 grainers my number of shots per kill dropped significantly.
Despite what you hear, they do not equate to whitetails. Mess up and they can really get your heart-rate cranked. I been there a few times - usually after dark and no fun. That is when the 870 with about 7 or 8 slugs in it got the call.
Hunting situations are more controled, but many times the bears come to bait at the end of legal light so you want to dump him right there.