It's just capacity planning - they can migrate me around as needed, though obviously they'd like to minimize the number of times that happens. Maybe there are periods where I spin up but don't get the full memory - that's OK, I'm only paying for what I use.
We have all of this virtualization technology but I still have to make hard decisions about size ahead of time. There are lots of providers that will let me burst CPU, I'd like to burst RAM. For people building small apps that get occasional usage, having more flexibility on memory would be a huge boon.
