In the present day, we're still arguing about equal pay for women in the entertainment industry and trying to ensure that men don't take home a bigger check for the same amount of work. This is not a new argument. Judy Garland was suffering from a serious case of pay disparity despite the film being completely based around her character.
Garland was paid a reported $500 per week, which in 1939 was no small amount. It was however dwarfed by the top two earners on the movie. Ray Bolger and Jack Haley, who were known performers and able to make stronger demands on the studio, were paid a reported $3,000 per week each. That's quite a lot now, but back then it was astronomical. It also seems wrong that the supporting cast were earning six times more than the star of the show.