In the same volume of air, you have less grams/moles of 02 present because the atmospheric pressure is less. This would mean that to have 14.7 lbs of air at a higher altitude, you would need roughly 15% more volume of air at a higher altitude. The 1 lb of fuel roughly stays the same as liquids are not effected in the same way as gasses. This can be proven with the ideal gas law: PV = nRT where R is the ideal gas constant, T is contant, and the grams/moles (n) stays constant. This means that V (Volume) is proportional to P (Pressure). So if atmospheric pressure goes down by 15% by raising the altitude, then the volume would have to increase to keep the proper proportion.
