How to delete messages on iphone?

On my phone it says messages are taking up 7.8 gb of data? I went into my messages app and deleted all of my messages and it still says that? And it says my iCloud storage is full? I don't even have my pictures on it. I'm so done with this apple bullcrap.

It probably archived your message rather than fully deleted them so try going on your messaging app and try and find under setting archive or deleted and then delete them again