Big Tin Cans

Can anyone fill all of the slots when designing ships? My designs are Big Tin Cans with the middle and bottom decks empty except for Armour or one or two items. It is not a problem although why mess with three decks if one would do.

But this might be a missed opportunity that could add even more interest to designing ships. At the moment weight is the main design constraint but why not volume as well. Mixing the capabilities of small heavy things with large light things would open up endless design possibilities. There also could be different technology trees to reduce size and weight.