New England is 5% of the US population and 12% of the MIT class, so NE is certainly overweighted at MIT (2.4x). California is 12% of the US population and 33% of the Stanford class (2.7x). So yes, Stanford is a bit more weighted to its home area but not by a huge amount.
It’s a bit of an apples to oranges comparison anyway because Stanford is the only top level private research university west of Chicago, so for a top high school student in the west it’s the obvious first choice IMO. Whereas a top high school student in New York, for example, can go to Princeton, Columbia or Yale, all within 90 minutes of home, and the entire Ivy League plus MIT within a few hours.