Perhaps a 50 seat RJ pilot makes half the dollar amount that a 737/320 makes, but he makes far more per seat/mile flown. That means ALPA gets more dues money per seat/mile flown for smaller aircraft. Same same a 777 pilot only makes 20% - 30% more despite carrying twice the passengers as a 737.
ALPA makes more dues money for more, smaller aircraft. Follow the money.