I believe a larger bear more likely to attack than a smaller one. They know how big they are & as you stand almost as tall or as tall as they are - ie: 6' to 7' for a good sized bear - their normal caution in sizing you up can or will make a difference. He won't attack in a predetory attack unless he believes he will be successful - normally. Hunger can make even man take chances he normally wouldn't consider.

When it comes to protecting cubs, size probably has nothing to do with an attack.