To answer this question, we would have to look back to a little Biology. So it is the auditory canal of the ear that is responsible as it is the channel from which the eardrum receives the sounds and then vibrates. The earplugs have to be placed in the ear canals so that their sound waves can easily travel to the eardrums and produce vibrations. So, the ear canal is approximately 0.3 inches in diameter and about an inch in length. But this is just an average size. And usually, earbuds are based on this standard size. But in reality, the size of everyone's ears and the ear canal differ. Therefore, this standard size does not fit everyone's ears so easily. So if your ear canal is too small, the earbud can't go deep enough and can fall out. Some ears also produce too much ear wax. So this can also be a cause for unfitted earbuds. Also, the spacing your jawline can affect the size of your ear canals. These joints are just placed in front of your ears and allow you to chew, swallow, and talk. Even a little movement in them can affect the size of your ear canal to a great extent.