My league has 6 players.

The Brawl has put us against a league with 51 players.

How are we supposed to have a fair brawl.?

Who makes these rules?

There's some algorithm that matches leagues, it's not done by an actual person.  I believe it's done based on their score in previous brawl tournaments, so it's likely the league you were matched against is inactive and doesn't score much in the brawls.