The Kestrel Constant. That's what I call it. Kestrel Tool describes it in their instructions but gave it no name.
Measure enough home made adze and crooked knife handles in the Pacific Northwest.
You would come to believe that the carver made the diameter to fit their hands.
Too fat or too skinny and you need a death grip to hang onto it to work all day on a pole or bowl, etc.
I learned that 1 1/8" or 3/4" were really the wrong size for my big hands and long fingers.
I buy just the blades and make up all the handles, couple dozen now.
I don't think that it matters too much(?) with a mallet but there is an ideal hand position to hold an elbow adze.
Of course, you can choke up on the head whenever you wish.
It makes a difference if you carve with an elbow adze for as little as an hour.
Strike at your heart rate but the stress on your arm becomes obvious.
I can switch hands at any time which helps ( I'm old now.)