[computator]

> Because what is in a 9 volt is 6 AAAA

But then the question would shift to: Why wouldn’t a AAAA cell have the same energy density of a AA cell? Remember that the argument is about energy density, not total energy.

By the way, I think that fsh’s reply is a convincing answer: it’s the extra packaging that kills the density.

[DanTheManPR]

The current collector also takes up relatively more volume, on top of the packaging (the wall thickness would be the same, and it's not a trivial part of the volume of small cell batteries).