ERF-NGC-European:
Actually, steering wheels did get smaller during that period, simply as a result of widespread use of power-steering. Only the German-built lorries retained full-sized steering wheels because at that time German C&U regs (equivalent) required that a driver should still be able to get home if the power-steering failed (for which you needed a big wheel for full leverage). IIRC, the same regs stipulated that you must have manual wind-up driver’s windows in case of electrics failure. Both entirely sensible in my view. Nowadays, of course, you are required to park and call out a fitter at enormous expense even if your radio fails to receive Radio 3. You mention DAF, which as you say seemed to retain a big wheel for longer than most - BTW, the difference between a DAF 95 and a 95XF steering wheel was huge! Robert
I never knew the XF but ironically the Merc 2534 also had a relatively smaller wheel,which I didn’t like,compared to the old SK.I just found that a larger wheel seemed to provide a more comfortable drive with a better damping effect in addition to the weird over gearing effects which the small wheel created which sort of defeats the object of power steering.
While at this time the Marathon was certainly an oddball in that regard with even the old Bathgate Clydesdale etc having a large old school type of wheel.
Which then changed to the later standard Leyland type shown here which seemed to be used across the Leyland range from around the late 1970’s and through the 1980’s in everything from 12 tonners to the Scammell Commander which again was a reasonably large wheel which I also liked.The advantages of the big wheel combined with power steering regards lightness and damping effect etc shown here albeit an extreme example.Trust me he couldn’t have driven the evil Marathon’s horrible steering like that and they might as well have given it a bleedin tiller type steering because that’s what the silly small wheel felt like to me in my hands. 
