Amazon only allows increasing the size of their AWS EBS volumes. While unfortunate, we can sympathize with Amazon, they’re doing it for a good reason (let’s leave it at that).
So, if you have let’s say 100GB EBS volume attached to your EC2 and you’re only using 20 GBs, modifying the volume to anything lower than 100 GB will generate an error: The size of a volume can only be increased, not decreased.
This essentially means you’re stuck with a large volume, wasting resources and paying large bills. The following article is a walkthrough of the process of getting around this issue and reducing the size of Amazon EBS root volume.