Thanks everyone, plenty more little nuances to follow up on.
I know 40GB is low (ridiculous, naïve etc etc), as noted, we had used streaming etc previously with 40GB and had plenty spare, so our usage is lower than most people.
Your situation reminds me of a guy I knew who bought a big old American V8 "tank" in the 70s. He didn't change his habits which included weekend trips to the Hawkes Bay. Except that now he could spend most of his weekly pay on petrol for that trip. He didn't do anything different: his car did.