I'm sure his musicals have contributed to that. But maybe he got a raise, too, when he stepped up to fill the gap after Richie left...

ETA: If I'm not mistaken, in Rich Bozette's book, he had pictures of contracts signed in 1986 that showed Tico was actually getting a bigger cut than David. I think both Richie and Tico got 1% while David and Alec got O.5% each.