At the risk of provoking another holy war over tyre widths - I think you have nailed a key point.
We get a lot of different kinds of mud over here, hardly any rock, sand or shale. (On some trails a lot of smooth river-rock type boulders). I find the relatively narrow 255/85R16 MT work well here. On some surfaces (hard packed clay) even these narrower tyres don't get a full tread depth bite.

