Heavyweights like Sugar Ray Leonard, Manny Pacquaio, Oscar de la Hoya, and Floyd Mayweather Jr?
N.B. Two of them fought for the largest purse of all time. One of them is the highest compensated of all time (though I am not sure if that is adjusted for inflation).
No doubt heavyweight is a spectacle and a draw, but when you look at highest compensated boxers and most watched bouts of all time, it spans a number of classes.
If people find entertainment value in the different weight classes--great! If the biggest purses went to lower weight divisions-- great! It means that weight division had particular popular appeal at the time and/or individual boxers were popular, and people were attracted to it with their eyeballs and tickets.
But I can hardly believe any decisions about purses were made based on assuming all the weight classes ought be treated equally. They clearly are not.
The point is, there isn't consistency in what athletes are paid and why. You're trying to apply criteria which you think make sense, but which likely don't add up in reality. Athletes petition for the wages they want to earn or feel they can earn. They may get paid that; they may not. They may think it is fair; they may not.
In the case of the Australian women's team, they argued for pay equity and they got it. Good for them. End of story. Whatever criteria you want to impose on the situation just weren't applicable this time around, and if we look across sport in general, we can probably find endless inconsistency.