I wanted to say that seems like a stretch but then I often find myself visualizing my best guess at the appearance, including species, of an unknown dog. It seems like size of dog and pitch of bark are negatively correlated
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.
