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.
What you say is true but I interpreted his complaint as being about the increased sunlight heating up the cab, rather than changes to insulation or air volume.
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.