There is a certain amount of storage that is assigned to your account that comes with your plan. It is a fixed amount that does not renew with each subscription, since the space is used up by the proofs and files from previous subscriptions that are still saved on your account.
How can I check available Storage amount?
You can check your storage amount on Account Settings page. The pie chart will also show you how much of your storage is currently used.
The Billing Administrators of your account will receive a warning email if the storage useage on your account reaches 75%.
How do I get more storage?
You can free up some storage by deleting old proofs and files from your account. Please note that items in Trash folder also count towards your Storage.