Hi there irir9,
Thank you for posting to Apple Support Communities.
"When you use Messages in iCloud, all of your text messages and the attachments you send and receive use your iCloud storage. You can make more space available when you delete texts and attachments—like photos, videos, and files—that you don't need anymore. "
From the image you shared, the 7.04 GB are only that of the top conversations. The 35.1 GB will include conversations as well as their attachments--the photos, videos, and files--that take up the most space.
This article has great information on how you can see that which is taking up space and delete that which isn't necessary to keep. Look under "Delete texts and attachments in Messages": Manage your iCloud storage - Apple Support
That should help. Let us know if you have questions.
Take care.