On the rest of your theory, Sic, I think part of it is simply that the Nebula
while sharing many of the same structural frames of the Galaxy
clearly seem to be using inferior internal components. It might be akin to building with merely cutting edge technology vs. with bleeding edge technology and as a result having to use more of the internal volume in efficient ways compared to the Galaxy
. Throughout TNG's run, we kind of got the impression that a lot of Galaxy
class systems were somewhat over complicated or untested, up to and including the warp core. It may simply be that as a "workhorse" design, the Nebula
is designed with less of the flash bang, and thus doesn't have as much internal volume to use, and thus had to make use of things like external pods.