It has more to do with the chosen jointing method. Dovetail lump guns need more space between the barrels to accomodate the lump, same for shoe lump. Wider in the back means the muzzles will more likely have some central gap, otherwise the barrels will look too steeply angled together.

Chopper lump needs less width at the chamber end and therefore is more likely to have touching muzzles. Monobloc will usually touch also.