I think it's because bigger dogs have larger throats and a bigger "voice box," so to speak. They have more of the vocal folds than a small dog, plus they have a

I think it's because bigger dogs have larger throats and a bigger "voice box," so to speak. They have more of the vocal folds than a small dog, plus they have a bigger chest and larger lungs to force more air through them. That's my bro-science understanding of it, anyway.