In the case of the March scopes with 56mm objective, the reason the main tubes are 34mm in diameter is simply that the thickness of the tube wall goes from 2mm to 4mm. This makes these high magnification scopes very solid. It works well for me as my March-X 5-50X56 sitting on top of my F-TR rifle has aimed around 20,000 rounds of match ammo over the last 5.5 years, without flinching. And it's still lighter than my NF NXS 12-42X56.
You have just given a bonofied reason for a scope of that design. If there is a real reason a scope needs to be a certain way to accomplish something specific. Plenty fine by me. I am sure you would agree a lot of other optics aren’t selected like that
