Ubiq:
If average family size was smaller 50 years ago than 25 years ago, there would have been a smaller annual increase in population (which would manifest itself in a smaller gender disparity 25 or so years later).
If gradually, average family size increased, we would see a gradual increase in population, and gender disparity for shidduchim.
You’ve decided to compare current disparity with that of 30 years ago. Therefore, comparing family size from 50 years ago with that of 25 years ago is totally relevant.