Different cab lift questions

Man, is this the truth! I was at a specialized gas company, and told the employee that I was going to fill up my small pony tanks (nitrogen gas cylinders) from their rented larger supply tank (and I said my tanks had the same PSI ratings as their large tank), and their employee manning the desk yelled at me and wanted the rental tank back because he said the supply tank was too big and would overfill the little tanks. He insisted it was size, not the PSI, of the tanks that mattered. I should’ve asked then what their big tanks out back were for.