Trends in space station design, weight versus volume

Here's an interesting table, showing the mass (in metric tonnes) and
habitable volume (in cubic metres) of various space stations, and hence
how many cubic metres of habitable volume you get for each kilogram of mass.

Now, IANARS, but I would have thought that in simple terms mass would be
very roughly proportional to surface area and so would scale as a
square, while volume would scale as a cube. Obviously this ignores
stuffing the habitable volume full of massive machinery, but I would
have thought the principle would hold as a first approximation of the
relationship.

Yet:

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

So ISS is 12 times as massive as Salyut 7, but only provides about 5
times as much habitable volume. It's three times as massive as Skylab,
but only provides 18% more volume.

The trend seems to be that the newer or larger the space station, the
poorer the relationship of volume to mass.

So what is going on here? Perhaps the modern craft are stuffed full of
more goodies (scientific equipment, coke machines, etc), but surely
1990's technology is more weight-efficient than 1960's technology.
Doubtlessly for electronics, but presumably for other things too.

I understand Skylab was exceptionally spacious, but there's four data
points here with a consistent trend.

Obviously there's some very unfortunate scaling going on which would
have ramifications for even larger stations. It also implies that for
the "space hotel" style projects, you'd be much better off launching 5
Salyut -style craft bolted together than one ISS-style.

Seems to me IIS really just a bunch of Salyuts bolted together (with the
odd CMG thrown in, valves for replenishment, etc), so why the
extraordinary difference in volume efficiency ?

Cheers

-- Ian