Amazon EBS are designed is such a way that they store data in volumes of a pre-assigned size that will be attached to an EC2 instance, similarly to a local disk drive present on your physical machine. Amazon EBS only allows the user to attach the volume to any other EC2 instance or also keep it in a standby mode without doing any read/write operation. This way it becomes important that once you have configured the volume in Amazon EBS you can’t easily scale it up/down.
Amazon EFS is on the other hand automatically scalable - this means that you need not to be worried about your running applications as there won't be any problems even if the workload suddenly becomes higher - the storage will automatically scale itself. Now, if the workload decreases - the storage will itself scale down, so that you don't pay anything for the part of storage that you don't use.