What Iowa's extra beam bought over Alaska

How much of Iowa's extra width over Alaska was used for torpedo protection and bulkheads?

A large amount, together with the space demanded by Iowa's heavier armour.

Iowa's extra beam points to a central difference between cruiser and battleship architecture. Beam provides room for layered underwater protection and internal subdivision. Iowa also carried a much heavier armour scheme than Alaska. These demands explain why two fast, long US warships could have visibly different hull proportions despite sharing some broad design ambitions.