Because the walls would get too thin. A block is cast with a certain amount of wall thickness. Example; ALL Pontiac blocks are the same external dimensions. They started out in 1955 with a 287 cubic inch and kept that block all the way through to 455 CID. The deck height at 10.24 and the bore space at 4.62 didn`t change. Nor did the head bolt pattern. There are no big and small block Pontiacs. BUT, that doesn`t mean one can bore out a 287 to a 060 over 455. One would have to have inch thick walls to do that.:shock: The block would be very heavy and would over heat with too thick of a wall. Course I left out the 265/301 blocks out. They were late, low deck bastard blocks that most don`t use.