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I think for the science of it, I think area covered, fan-base spread and available wealth would be important factors determining the optimal size of a football league.

The smaller the area, the more teams can be supported due to overall decreased travel costs (of course this applies for the total number of teams before they become regionalised and may extend beyond the league, such as England hitting 116)

The better the spread of fans too, the more teams you can have. Scotland has I think 5 teams with an average attendance over 10,000. England has I think 40+. This is why Scotland only has 12 teams despite probably being able to host a decent few more is the lack of support.

Available wealth. This covers three-bases. Is there the money for newly promoted teams to be viable, is there money available to make extra matches profitable and also are all the clubs rich enough for it to be even. If there is money in more teams and they can be competitive, more teams will likely be added.