If you compare the pressurized volume of all the space stations launched to their mass, their volume-efficiency (in this context volume divided by mass) seem to decrease over time.
In chronological order:
Salyut: (actually seven stations, but practically identical. Salyut 1 numbers are used)
- Pressurized volume: 99 m³
- Mass 18,425 kg
- Cubic meters per metric ton: 5.37
- Pressurized volume: 319.8 m³
- Mass 77,088 kg
- Cubic meters per metric ton: 4.15
- Pressurized volume: 350 m³
- Mass 129,700 kg
- Cubic meters per metric ton: 2.70
- Pressurized volume: 916 m³
- Mass 390,378 kg
- Cubic meters per metric ton: 2.35
- Pressurized volume: 15 m³
- Mass 8,506 kg
- Cubic meters per metric ton: 1.76
What is going on? This is quite the opposite of what one would expect from technological improvements.
What is the reason for the volume-efficiency of space stations dropping when the overall technological progression improves?