More successful in the past doesn't mean bigger.
The only sensible way to define the size of a football club is by its current ability to sign the best players.
The biggest clubs have a gravity that relates to their size. The best players are always drawn inexorably towards the biggest clubs.
money mainly
Like it or not , clubs are perceived by size throughout the whole pyramid. If money is the same then players will choose clubs on size as well. Lower leagues especially.
Some people hate this argument but I quite like it.