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. The ideal station, from this point of view, would have very low
frontal cross-section (in the limiting case, some kind of cigar shape
pointed in the direction of the orbit).

Volume is an imperfect proxy for frontal cross-section but I suppose
they would tend to correlate somewhat.

High air drag is one of the problems that most shuttle external tank
based proposals had.