Mike, Wide low pressure tyres off road have less rolling resistance than narrow tyres at a higher pressure, especially on a rigid or hardtail, less noticeable on a susser.
The tyre manufacturers did a fair bit of research on it a while back, quite alot of info on the interweb, Schwalbe were one of the pioneers I think.
The reverse is true on the road, but it's smooth there unless you live near me where potholes are the norm 🙂
It's all to do with a wide footprint and deformation over small bumps.
I'll find a link to all the reasearch and post it.
The key is the low pressure though, which to do properly you need wide rims otherwise you get lots of tyre squirm which is a waste of energy.