There are two parts in this question: 1. Tom Seaver is a pitcher, one of the best starting pitcher in base ball. 2. When you say "A, eat your heart out!", you are actually telling A (who is a famous person) that you or someone else is going to outperform him. So, in this case, there maybe another pitcher who is pretty good, perhaps as good as Tom Seaver. You are actually (jokingly) telling Tom Seaver that he is losing out to that pitcher.