The reason this feels non-obvious is that people like to think that they're choosing a variable number of small slices of a big datacenter, scaling up and down

The reason this feels non-obvious is that people like to think that they're choosing a variable number of small slices of a big datacenter, scaling up and down hour-by-hour or minute-by-minute to get maximum efficiency. Really, though, you're generating enormous overhead while turning on and off small slices of a 128-core monster with a terabyte of RAM.