What I expected to happen
I would be allowed to fill an area of 32768 blocks, as it specifies that I should be.
What actually happened
I was not allowed to fill an area of 32768 blocks, even though it specified that I should be.
How to reproduce
Enable cheats.
Type
/fill ~-15 ~-15 ~-15 ~16 ~16 ~16 minecraft:air
into chat
Possibly what is causing issue: '0' position is not being used. If the container has 32,768 positions going from 0 through 32,767 and you skip the 0 position, you have only 32,767 usable positions.
I would test by attempting to fill a 33x31x32 area (32,736 blocks) and also a 151x31x7 area (32,767 blocks). Just trying different areas and seeing exactly how close you can get will help reveal how large the container really is.
90x28x13=32,760
32x32x31=31,744
32x32x16=16,384
31x33x16=16,368