It can be surprising for folks that athletes have a lower resting heart rate than others do. I had thought it was greater efficiency in blood to muscle delivery, but I don't know why I thought that.
Though, if you Google heart hypertrophy, you'll find a bunch of scary things, because your heart can also get bigger in a bad way rather than a good way.
If you have links on any of this, it feels like it would be a very fascinating read.