We don’t have anything absolutely quantitative, since we migrated to Kotlin over time. While we were converting, we were also adding new features — so the code was moving a lot. It’d be difficult to get a 1-to-1 comparison of before and after code.
FWIW, anecdotally we had about 15–20% fewer lines of Kotlin than we did in Java. And qualitatively the code’s readability and clarity were much improved.