[jasone]

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.

[ZiiS]

It is more about the pulmonary strength needed to start the bubble in sufficent glass for the bottle size.

[Arech]

I'd guess that it's also relates to a pressure that glass blower had to create. Then the total volume would be less than free volume.