Different sized wheels on different axles

I spend a lot of time tramping up & down the M80 just now. As a result I pass a lot of HGV traffic, including the tankers from Grangemouth going to the Glasgow/Lanarkshire/Dumfries area. Something I've noticed is that on the tankers, but not on any other HGVs is that the mid axle seems to be a lot smaller than the others on the unit. How come? Every other 3-axle unit seems to have 3 rows of equal diameter wheels, but not the tankers. It's not just one manufacturer either - last night I spotted Scanias and MANs with similar configuration.

Any idea why?

PS - I've noticed the odd car transporter with uneven sized wheels but they're usually weird shapes/sizes anyway - the tankers just look like normal units with day cabs, pretty much the same as any supermarket short-haul machine.

Pretty much, though usually a lot less blingy. On a lot of them the wheel arches seem to be unpainted chequer plate.