Did you click "Review Large Attachments" to see what most of that is? I'm thinking you have open text messages and someone has sent you 1 or more whopper "attachments."
And easy resolution is to close all conversations and start new ones when you want to text people again. Many just leave texts open forever and thus pile up huge amounts of data-hogging images/videos/music/documents people send to each other. Much like hanging up the phone at the end of a phone conversation, close a text conversation when done and then "call" them again next time you want to start a new text conversation.
In purging what is sometimes very long threads, you'll delete whichever one has the 1+ huge 'attachement(s)' in it.
And since there are differences between the 2 devices, I'd first delete all text messages, then log out of both devices iCloud, then back again on both devices to get them synched back up. It seems your iPad is operating separately from your phone. If it's not that, my guess is whatever attachments are hogging up iPad space are openable on iPad but not on iPhone. Or maybe iPhone storage is too close to "full" but iPad has the room, so it is "accepting" the big attachment(s) while the iPhone can't, so it leaves it in iCloud.
