I've always assumed that the average quality is based on your 14 best players (however big your squad is). I believe the game is based on 11 players plus 3 subs. We know that any team with less than 14 players are either made up to the required number by the GE, or else the team will forfeit their match.

Therefore, a proper comparison between teams with different numbers of players could never be made. So as the minimum number is 14, I believe this is where the comparisons start.