Some volume calculations could be done too. There might not be room enough in the pyramids for a seven year famine food supply even if Joseph had arranged for them to double as storage areas.
According to the article I read (which may be incorrect), he said they were built to store the grain, not re-purposed. You are correct, it would be easy to estimate volume required if we knew population size for the Nile river valley and consumption per capita in volumetric units.