iCloud back up inconsistencies

WWhen I go to back up my iPhone 6 (ios8.1.3) to the iCloud if tells me that the back up will take 29gb, fair enough - I have a lot of photos and videos on my phone.

However when I do back up my iPhone it tells me that it's only taken up 1.2 gb in my icloud. It's not due to lack of storage on icloud as I have the 200gb option.

Any help or ideas? I'm rob to make sure that everything is backed up because I want to update my iOS to the latest version but want to make sure I've got everything backed up just incase anything goes wrong.

Thanks