Having the right bit on each horse is crucial. You have to experiment to find the right bit for each horse, and the better horseman you become the better feel you'll get for each horse's mouth.
When it comes to proper fit of a bridle, you need to consider not only the length of the headstall, but the proper adjustment of the curb strap. Generally speaking, I like a bit to be snug in a horse's mouth. I don't want a bit to be hanging too loose in a horse's mouth or it'll be completely ineffective.
If a bit is too tight or too severe for that particular horse, he'll have a tendency to throw his head up or gap his mouth open when you take ahold of him. If it's really bad, he might even rear.
If you've got too light of a bridle on a horse, you have no control and you're going to have to try to rely on your strength to get him slowed down and stopped. I don't know many people who are going to win a pulling contest against a horse.
