It has nothing to do with belly breathing.
The more developed and powerful the heart, the greater the volume of oxygenated blood it moves per beat, and therefore the less often it has to do it.
The tour cyclist Miguel Indurain had a recorded resting heart rate of 32 beats per minute.
I myself used to run at 48-55 pbm resting heart rate, but after the long sedentary period enforced by injury last year, it went to 70.
I now run at around 60, and will bring it down further.