the competition size is based on the sizes in real live. the differance in matches is max 9 matches. because the smaller competitions play 3 or 4 rounds in stead of 2. see also discusion in:
http://rockingsoccer.com/en/soccer/forum/home-en?search_content=12%2014%2016&topic=256
For example:
"There are at this moment leagues with:
10 clubs => 4*9=36 matches per season (Each club play’s all others 4 times)
12 => 3*11=33
14 => 3*13=39
16 => 2*15=30
18 => 2*17=34
20 => 2*19=38
So it is true that leagues with 16 teams get less matches, but as Vincent said: Only 20 matches count for XP for players. So with 30 matches you can give 21 players a full XP boost by playing matches.
30*14=420
(matches * max. players per game = max ammount XP rewarded)
420/20=21
(Max amount / countable matches = max players with full XP)
With play testing we never got more than 16-17 players you would do that for, because other players are considered to weak to play, if it could be helped."