Google's storage offerings are fairly generous, but there are limits. Google is happy for you to pay extra, but many people prefer to work within the existing 15GB limit. What happens when you fill up that space?
As Google Operating System notes, Google won't actually delete files when you hit the 15GB barrier, but it will affect your use of Google services almost immediately. Rather obviously, you won't be able to upload any more files, but above that, because the 15GB Google offers is shared across all of its services, your Gmail account will also suffer, because you won't have the storage to process new messages. To solve that, you will need to delete something.
One nice factor here is that Google doesn't count smaller photos or videos stored in Google+ Photos. If your snap is under 2048x2048 or shorter than 15 minutes, Google says it doesn't count towards your storage limit. As we've previously noted, it's simple to keep track of which files are eating up your Google storage quota.
There was a time when 15GB of online storage sounded impressive, but many of Google's competitors now offer terabytes of free storage by way of comparison.
Google's Storage Problem [Google Operating System]