In Hollywood, there are no rules when it comes to determining how much an actor should be paid. As long as they're given the legal minimum, it's every man (or woman) for themselves at the negotiating table, meaning that salaries can end up being higher than expected, but can also come in on the low side too.
It's also tricky to determine which actors are more valuable to a project. You might think that the person at the top of the poster or the actor playing the main character is the one who deserves the biggest amount of cash, but in reality, the face of the movie isn't guaranteed to take home the biggest cheque at the end of the shoot.
In fact, it's not unheard of for supporting actors to be the ones who get paid the most - even taking home more than the leads. Maybe they're an esteemed character actor with a legacy of famous work behind them and the star of the movie is a fresh-faced newcomer, or maybe both actors are on even footing in terms of fame, and it's left up to other factors to decide who gets the largest payout.
Either way, the phrase "work less, earn more" definitely applies here...