iCloud storage always full

Solution 1:

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.)

Language

Settings for Mail, Calendar, Stocks, and Weather

Time Zone

Solution 2:

Go to Settings > General > Storage & iCloud Usage. Under the iCloud section, tap Manage Storage. This will show you everything that is using space in your iCloud account. If you see something you do not want backing up to iCloud, then disable and delete it here. iCloud Photo Library may be the culprit.

On the bottom of this page you can also select Change Storage Plan to purchase additional storage; in the US, the $0.99/month plan gives you 50 GB, which should be plenty for you.