Smallbore made some valid points, to which I will add one:
the 16 has the best outside dimensions that give it the ideal bulk for easy handling and manipulation.
I have handled 2 inch boxlocks from Alex Martin, William Ford and a Holland sidelock. As one poster said, you really must handle one of these to realise the full meaning of good handling. It is not the lightness, a Baby Bretton is lighter, but the overall live feeling that is unique.
But.... I have handled a single Midland 16 bore outside hammer that can rival the best 2 inch 12. There is also a 16 gauge Holland hammer which a friend has that has that live feel to it.
The difference is that you can walk into any shop (almost) and buy 16g shells, the two inch is a [censored] to find.
