[spookie]

From an empirical assessment, volume has gotten bigger. Part of it stems from the devices getting bigger as well. However, I would wager energy density hasn't improved as much as the increase in volume, given that devices got bigger first.

I think this can also be corroborated by the fact that there are less and less small smartphones options. And while there are some, they're plagued by poor battery performance. See latest iPhone SE.

There's indeed, by this logic, reason to believe less space taken by other components lead to better battery life by the sheer volume available as a result.