Specifically the "Now Playing" screen. Before the cover of the album was a transparent background, but they instead made it a standalone picture on the right side, shrinking to forward and back controls (and in my podcast app, shrinking the skip 30 buttons).
Do you mind elaborating? I'm also running CarPlay with iOS 13, and the apps all seem to have the same usual button sizes/target areas.
I suspect you're thinking of the new "dashboard" view (pictured at https://www.macworld.com/article/3403343/everything-new-in-c...), but even in that case I haven't run into a scenario where the buttons/targets seem smaller.