You have to compare by energy per user or per interaction. Lemmy would cost a ton more and consume a lot more energy than reddit if they were the same size. I don’t think Lemmy instances would even be able to handle federating with other large instances at that size and the fesiverse would get split. It would start needing sharding and replication, and some kind of consensus algorithm to choose where to get most data, and you’d have the same problem cryptocurrency has.