It’s really a question of what you want to *do* with the ammunition.

And ofc the 290 mm thingamajig isn’t really a gun at all in the normal sense anyway. To wit: http://en.wikipedia.org/wiki/Mortar_(weapon)#Spigot_mortar
A certain inefficiency in terms of payload is apparently an inherent characteristic of the design; the gain being the weapon itself being ligher and smaller.

In any case your point is at best partially correct; it would be more accurate to state that larger caliber correlates to larger explosive payload ergo power, *all other things being equal*. Start screwing around with radically different designs and of course the correlation goes out of the window.

And wow most those 152 shells had surprisingly small amounts of explosive in them. The HE ammo that the ingame kv2 uses carried 5.47–6.86 Kg of explosive, compared to the Brit stuff that’s tiny.