Moment of inertia isn't rotational; it effects how your ship gets knocked around by impacts from missiles, shockwaves, other ships, etc.
The Mars model was made back before Bob had the MoI code figured out. Quite a few mods from that period had weird behaviour relating to the MoI code.
Granting this is speculation, but here's what I think.
The B5 model is newer and has proper MoI for something of its size: that's why it blows up so rapidly. The Mars model doesn't have proper MoI; it's probably copied from a large stock FS ship, so it blows up at the same speed as a large stock FS ship...which is very slow for a planet-sized object.