I've heard people talking about Pioneer calabashes being on the smaller size bowl wise, I know they were popular in America at one time.Fair enough. I thought that pipe resembled the one Calabash I have, which does have a fairly small chamber in relation to the stummel.
The size groups are kind of a loose description. They can range from 35.56mm to 44.45mm to some, but to others can vary. Then there are monsters like this with depths of 76.2mm that the only class they have for are magnum.That's my biggest bowl briar so I used it as a comparison. I don't know what a group 4 looks like so I can't say if these are bigger or not.
I love that pipe. The shape, grain, colour and stem work perfectly together.