Why are 40 pin and above DIP packages generally wider than 28 pin and below
First, some of the older chips really had a large enough die that simply wouldn't fit in the narrow package, and chip complexity often correlates with pin count. Even if the die shrinks a few years later (when better technology becomes available), newer chips would still use large package for compatibility with older designs.

Second, the leadframe around the die gets more and more dense as the number of pins increases. At some point, the leadframe itself calls for a larger package, even if the die would fit in a narrow one: