While the role of Superman itself is surely a bigger one, Superman is the biggest name in the history of comics, Wonder Woman isn’t all that far behind Superman when it comes to name recognition.
Gal Gadot was a relative unknown prior to taking the role of Wonder Woman. She certainly wasn’t a “movie star” capable of demanding tens of millions of dollars out of the gate.
The problem is that the same thing could be said of Henry Cavill when he signed on to star as Superman in Man of Steel.
One would think the situations of these two actors are fairly similar, and such the paychecks would at least be closer together.