ICloud won’t let me back up, even with full 5.0 GB available

What to do when you do not have enough storage because your backup size is too large:

• Delete the current backup on your iCloud for that specific iOS device.
• Your iCloud backup is now automatically turned off, so turn it back on.
• A new backup is made and has a reasonable size.

Theoretically your phone could crash in the short while you do not have a backup, so you could run an iTunes backup first.

Unless it has anything to do with Apple Watch? I can't see or find anything about activity app being backed up the cloud, but that's the only thing left I can think of

The extra backup space is definitely from the watch.

https://support.apple.com/en-us/HT204518

Here's what your Apple Watch backup will include:

App-specific data (for built-in apps) and settings (for built-in and third-party apps), such as Maps, distance, and units

General system settings, such as your watch face, known Wi-Fi networks, brightness, sound, and haptic settings

Health and Fitness data, such as history, achievements, Workout and Activity calibration data from your Apple Watch, and user-entered data (To back up Health and Fitness data, you need to use iCloud or an encrypted iTunes backup.)

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