It may sound simple...but a big bear will look big! there should be little "what ifs" or judging required...

But I agree the ears will tend to look small for the bears head...

the head will look big...though may look small compared to the bears body...

less space between the bear's belly and the ground...

I think when you're judging a small bear vs a medium bear is when these factors come into play more, but like I said before, you should know a BIG bear when you see it!