Horizontal scaling brings down total compute use if you have many distinct uses. Imagine of your only unit of compute is a single bulky machine. You don't full

Horizontal scaling brings down total compute use if you have many distinct uses. Imagine of your only unit of compute is a single bulky machine. You don't fully saturate it, but you need a second machine to avoid downtime anyway. Now you spin up a second or third service and suddenly you need 5 or ten machines and your compute utilization is 20%. You can pack things in tighter. But then you have a knapsack problem, and that's easier to solve efficiently with many small blocks even if it costs you a 1% overhead or whatever.