erm, i would guess the 401 *has* to weigh more, although perhaps not much. piston is aluminum, cylinder wall is cast iron.
CI weighs more per volume than AL does.
therefore, every thousandth of bore reduction means less Alum and more CI. even if the cylinder bore in the smaller displacement is cast to a slightly smaller dia, you still have more CI in the deck.
therefore, if the casting is same in almost every other respect other than bore size, they would have to take CI out of some other part of the casting in order to maintain weight.
worst would be if they just used the same casting but simply bored the 425 extra large.
i'm certainly not expecting it to put a 401 into the mid 700s, but it's just something i'm curious about.