Why does the armor type affect ship maintenance cost (significantly)?
I've done some brief tests, to verify the effect and get an idea how big the diffenrece actually is: (just changing armor)
Clean Hull:
Krupp III: 5.118.653 $/m
Modern II: 7.728.106 $/m
Increase: ≈ 51,0 %
Expensive Design (just for testing; not overweight for both types):
Krupp III: 37.768.240 $/m
Modern II: 49.138.930 $/m
Increase: ≈ 31,0 %