Nerri, did you ever run that experiment?OK ..
IF the air in that 2L bag cooled at all a decrease in volume will also be observed. Charles' Law.
But if O2 if truly being consumed as rates like Dr Ace explained using Stoichiometry ( I love saying that word ) - just fancy name for recipe calculations for Chemistry.
then the volume of gases in the bag would decrease as well.
I have O2 sensors and I can actually run this experiment.
I'll keep you posted.