On 22 Feb 2008 01:11:32 -0500, in a place far, far away, Jim Kingdon
made the phosphor on my monitor glow in such a way
as to indicate that:

Mass Vol m3 per kg
Salyut 7 20 90 4.5
Skylab 76 361 4.75
MIR 124 350 2.82
ISS 246 425 1.72

It is hard to prove anything from only 4 data points (which have
hard-to-control-for differences such as nationality, whether the
purpose was quick-and-dirty or "optimally" designed, etc), but if I
had to guess, I'd say that people have gradually figured out that big
stations have a lot of air drag and thus require a lot of reboost
fuel.

Only if at low altitudes. This is a function of launch costs. If
costs of reaching it weren't a consideration (and earth observation
weren't an issue), a station would like to be much higher than ISS is,
with an upper limit as it gets into the belts. The altitude is
constrained by the increasing reduction of payload to reach it with
the Shuttle (a problem that could be solved by a station-based tug).