Storage requirement when using for AWS ECS
This is how the ECS optimized Amazon Machine Image (AMI) is set up, with a separate volume of that size. You can read more about it in the AWS docs. You are completely free to create your own image that has Docker installed and runs the ECS Agent Docker container.
Take into account however that Amazon choose this setup for a reason. Docker images take up a lot of space and logically separating that space from the OS space makes sense. You can debate about 22 GB, but on the other hand; the cost of such a volume are very low on AWS.