Have you ever worn clothes that were too loose or too snug? Remember that uncomfortable feeling you had the entire time? Or carried a backpack that almost sat on your rear, maybe worse when it was hiked up to your neck and you couldn't take it off without intense assistance?
Horses experience the same thing. Saddles slung too far forward over their shoulders that hurt. Girths strapped too tightly over several folds of skin because it is sitting too close to his elbows. The bridle overtightened creating a unique smile on the horse's face thanks to the bit pulling on the lips. Halters that are so tight they choke the horse's throat, or too loose that the noseband could be pulled over the horse's muzzle and off completely.
There is a proper fit to every piece of equipment we use with the horse – not just to prevent injury and irritation, but so that this equipment is able to work as we want – to convey the communications we are giving the horse clearly and with the least amount of distraction possible.