Horses ears are wonderful things.
A delicate set of muscles surrounding the ears allow the horse to express emotion and focus hearing to specific locations. Their ears have evolved to be one of the horses greatest assets for survival and communication, they have 10 muscles per ear to independently rotate the ears to locate multiple sounds and thus potential predators and indicate their feelings towards herd members.
The picture indicates the superficial structures the bridle would lay over and you can see the volume of muscles, tendons and glands in this area, not shown is also the delicate but influential fascial layer. Its important to check your brow-bands and headpieces aren't too tight so they don't add unnecessary pressure to these structures leading to discomfort and restriction of natural movement of the ear and poll area. Although it would be hard to measure or test i'm sure its comprehensible that horses can have headaches, like you if you've borrowed a riding hat that is too small for you.
The junction of the browband and head piece sits right over a delicate glad area of the lateral throat. Its common for a too tight brow-band to pull the headpiece into the base of the ear, as well as buckles taken up too high (often when extra holes are made to fit a bridle too big for a small head) will add additional pressure into the soft and mobile area of the throat.
You want to avoid muscle and fascial hypertension as a consequence of the horse tensing against discomfort. Your horse may express this discomfort with signs such as head tossing or not wanting to soften into a contact or even evading the bridle when approached to be tacked up.
Ensure your horses ears move freely whilst wearing his bridle, look out for any rubs and listen to him if he shows signs of sensitivity or behavioural and performance change.