Well in the 747 it appears that the air conditioning system lives above the false ceiling, but there's still a lot of empty space up there.
And, yes the A380 does have a kind of egg shaped cross section-I'd forgotten about that.
It's just that in the DH Comet, the below floor/belly area doesn't appear to be used for anything (fuel?), whilst the cabin goes to the full height-so there's no set cross section.
Anyway, where were we? Propulsion?