Airtightness is really a property of the building enclosure, exactly analogous to moisture permeability and U value. It doesn't depend on the volume, but does depend on the surface area. The air permeability of materials is typically measured as a flow per area at a given pressure difference across the material. U value is similar. If we want a metric to use to measure the airtightness quality of the exterior enclosure of buildings it just makes sense to use something very similar.
Here is an example to help demonstrate how volume is not proportional to surface area:
Comparison between ACH50 and CFM50/ft² for a 2000 ft² home at 3 ACH50
House Is 50 X 40 X 8
Volume = 16,000 ft³
Surface Area = 50 X 40 X 2 + 180 X 8 = 5440 ft²
CFM50 = (3 X 16000)/60 = 800 CFM
CFM50/ft2 = 800/5440 = 0.147 CFM50/ft²
Increase height to 2 story at 3 ACH50
House Is 50 X 40 X 16 Volume = 32,000 ft³
Surface area = 50 X 40 X 2 + 180 X 16 = 6880 ft²
CFM50 = (3 X 32000)/60 = 1600 cfm
CFM50/ft2 = 1600/6880 = 0.233 CFM50/ft²
In this example, when the volume doubled, the surface area increased by 26%. And when the ACH50 stayed the same, the CFM/ ft² of surface area increased by 58%.
The use of Air Changes per Hour at 50 Pa (ACH50) started at least 60 years ago by researchers who were interested in ways to predict the natural infiltration rate of buildings, which at the time was most commonly measured in Air Changes per Hour. The use of this metric when studying air quality in buildings does make some sense. If a pollutant is suddenly released in a building, the time for the concentration to decay by a certain percentage does depend on the infiltration measured in air changes per hour. The analysis of a tracer gas decay test gives a result in air changes per hour. So when they started measuring airtightness, for use in estimating natural infiltration in air changes per hour, it made sense to use ACH50 as the metric. But two homes with the same volume can have very different surface areas and the purpose of measuring is to determine something about the construction quality. We think ACH is the wrong metric.
Many standards are now trending in the direction of using square foot of enclosure area instead of ACH. Examples include US Army Corp of Engineers, LEED, Swedish and US Passive House.