ShiroKuro There is a way to see a breakdown of what content/app is taking up the how much space in your iCloud storage. On your iPhone, go to Settings -> Click your name at the top -> iCloud -> Storage. As an example, mine is showing iCloud Photos taking up 150 GB, iCloud Drive 91 GB, and Backup 72 GB.
If your iCloud Photos is the largest, the most likely culprit is videos in your Photos library. Open Photos app, then select "Collection" (bottom left), then scroll down to Media Types, click "Videos". There will show all videos in your library. You can multi-select and delete from there.
iCloud Drive usually contains all files in the "Documents" folder on your Mac (if you use one).
Backup, if you noticed the size of mine, can be surprisingly large if you have multiple Apple Devices. In a pinch, it is safe to delete backup of a device to reclaim some space. Since all documents, calendars, contacts, notes, reminders, and photos are synchronized to iCloud already, what backup really does is to ensure the app settings (and maybe a few local data like chat history) are preserved when you need to restore the phone. If the backup schedule is daily or even weekly, deleting backup just means you won't have it until the next day or week your device is backed up again.
