Gotcha! Looks like I need to buy more storage.
You can also disable certain apps from backing up to iCloud. For example, you may not need all your magazine subscriptions and back issues to back up to iCloud, taking up space. You can turn those off.
I've turned off Navagon's backup to iCloud. There's like 700MB of additional downloaded maps that are stored on my phone, but I can redownload them at any time so I don't bother having those backed up to iCloud.
But, I will say that while 5GB may be enough for an average user with one device, it runs out very quickly if you take a lot of video or have more than one device, say an iPhone and an iPad.
My wife, who isn't really all that tech savvy, seems to store a ton of data on her devices. She's currently using around 16.5GB to store backups of her iPhone and iPad.
She's upgraded to 25GB for $20/year and I think that's totally worth it.