Why were the TWA TriStar middle seats smaller than the aisle seats?
Those seats aren't smaller! TWA had fold over seatbacks, resulting in a nice table for the seat next to you if the flights weren't full. They are the exact same size as the other seats. Also, those things down the centerlines between the seats were coat closets.

Later replaced by a full width seat when they went from 2-4-2 to 2-5-2 in their last configurations.