Fixed costs are amortized. The block's footprint is spread across transactions. Arguing otherwise is like someone taking the power bill of a bank to infer the cost per teller-window transaction, and the manager arguing that teller-window transactions don't burn energy, branches do. Yes, sure. But also irrelevant.
And yes, if the branch tripled in size its energy use would reduce the per-teller energy footprint. (Assuming constant transaction volume.) But that's a hypothetical.