..just partially copied something favorable.
Most of my boxwood chisel have a symmetrical or close to symmetrical pillow shape (round, but pillow). I leave the tops a little fat. It's nice if the taper to a smaller diameter where you're holding them so that the thin bits are in your pinkie and the "pillow" shape fills your hand a little better as they bow out along the length.
They stick in a loose grip better than a straight handle - I just had to pick one up to see what they fit and why that shape became a favorite as I have some distaste for logical arguments in something that just "is" by feel. It tends to lead to "logical improvement" without further testing of what is.
(I just walked out and checked the various styles of handles and all of the older chisels that I have tend to encourage a handle grip with about an inch of handle sticking out of the web of the hand - word on this shortly - and the lower pinkie and ring finger in a lower diameter spot, either at the ferrule or slightly above).
Those fingers close a small diameter and allow not having to grip a chisel very hard to keep it in place moving it or while hitting it.
The extra inch at the top over the stanley style handles is to keep from hitting the web of your thumb no matter how much the chisel slips. Most people probably think my handles are too long. Most people probably think the chisels in general are too long - they're close to the longer english chisels I have. I hit the web of my thumb on a short handle once, and it was memorable.
(on japanese chisels, a handle grip puts the pinkie around the ferrule or down from it a little bit, so even though the handle looks almost straight, the same kind of hand thing is going on).
Exploration of the handles in the cabinet does show that some of the older ones remain fatter toward the end and not all have closer to pillow symmetry.