My account is unexpectedly full

Sierra can make use of iCloud to store your Documents and Desktop folders in iCloud Drive so they are available on all your devices. You may have enabled this when you installed Sierra. While useful, this can eat up your iCloud space. To switch it off, go to System Preferences’ iCloud pane, click Options next to iCloud Drive; uncheck Desktop & Documents Folders. Empty versions appear on your Mac; drag items back from those in iCloud Drive.