Correlation not cause. But you said you're just messing around, so you probably already know that.

Imo the budget isn't simply a multiplicator for the revenue here. I'd say that because a previous movie was successful, they have a growing and already interested audience for the second movie and the success also encourages them (and allows them) to spend more on the sequel.

So in short the success of the previous movie and the growing audience cause both the bigger budget and the bigger revenue of the next (which makes the two correlated but not causal).