In dual mono, why does 200 become 400, but 250 becomes 800?

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I can understand why the casing differences allow a 250 to deliver more power output than a 200 (with notionally identical electronics).

But surely the question is: why does the output more than double for the 250 when in a dual-mono configuration?

ETA:...which is the same question Hifi_swlon asked, the other way round.