750 ml has nothing to do with how much wine people are expected to drink at once; it's an artifact of glass-blowing. A typical medieval glass-blower's lungs allowed them to create bottles of 700-800 ml with a single long breath.
Is there something about glass blowing which makes it air-inefficient? My vague recollection from my time as a low brass player in college is that the prof's lung capacity measuring apparatus (yes, really) put most of us in the 3-4L range, with one outlier around 5L. Granted, we were trained for such magnificent feats of blowing hot air, but so too must glass blowers be.
