It seems to me that metabolic state should also be taken into account.
A lower respiratory quotient for fat metabolism means expelled CO2 is much lower when utilising fat for energy (CO2 litres per kcal [0]), making respiration easier and requiring less tidal volume. A lower CO2 concentration in the alveolar space means O2 can be more effectively taken into the blood [1].
Hormonal disruptions also seem to play a role [2].
[0]: http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/metab.htm...