In my old Tanker days on a large trijet, we had fuel tanks fitted in the cargo bays. Weight of fuel in those tanks was ZFW. If we were were operating in a pure AAR role MZFW was often the limiting factor. The MTOW was well above full tanks and performance was normally not an issue either depending on where we were based.
Using the same airframes for Air Transport we would put a couple of hundred squaddies and their kit on board which would be well below MZFW then fuel for the trip. So long as we didn't use the tanks in the cargo bays. This could, depending on the leg be MTOW limited.