I'd say its good for both.
Small sites because it is low-bandwidth to figure out how to use. Time is money and if I'm just tinkering on the weekend I don't really want to learn Kubernetes or the labyrinth that is AWS; I just want to ship an app.
Big sites because your users get routed to the node closest to them and again you can do this without a lot of time investment. For Elixir I just wire up Libcluster and my nodes can talk to each other.
I really want GPUs on Fly soon though. Just take my money Kurt. (I hear they're working on it)