Actually, the size of ears plays less of a role in your ability to hear than the shape of those ears. Human ears are specifically shaped after millions of years of evolution to pick up the sound waves that are most important to us… mainly, the vibrations a human voice makes. Some people have better hearing than others, yes, but that is less dependent on their size.
So Sam might be able to hear a little better than Dean, just like it’ll be easier to deafen him by shouting, but it’s not a huge difference, and there are likely regular sized humans who might be able to hear just as good as Sam. There are a lot of people out there who can hear higher pitched tones while others can’t hear a thing.
Brothers Apart 