A larger but well insulated cab could have a lower A/C energy cost than a smaller but less well insulated cab. More air volume doesn't dramatically change the

A larger but well insulated cab could have a lower A/C energy cost than a smaller but less well insulated cab. More air volume doesn't dramatically change the energy footprint when a stable temperature is being held for a long period of time. With sufficient insulation, the main consequence will be greater hysteresis, i.e. bringing the cab to temperature might consume more energy.