I'm using EU formula but, as Warboy said, it doesn't matter for explosives.
So the cause of the high damage is the 2x2 size, which causes the unit to be hit *4* times by area-type damage:
- explosive
- fire
- stun..? (AFAIK stun uses the normal damage formula, so 0-200% in EU, but still "degrades" like explosive over the area it covers).
If this is intended (IE: not a bug), then I'll balance-tweak armor damage modifiers for 2x2 units, accordingly to this behavior.