IPad becoming full of storage space instead of empty?

My iPad has been telling me for a long time that my storage space will soon be full. That's why I started to clean up my mail program by resending old but important emails to me, backing them up on my new laptop, before finally deleting them on the iPad. These are mostly emails with larger photo attachments.

As soon as the emails are backed up on the laptop, I delete them on the iPad, but the iPad's storage space is getting smaller and smaller instead of larger than expected.

What shall i do? I don't understand the logic of my iPad and I'm about to hit the wall. But before I do that, I'm hoping for support from someone who understands the soul & logic of iPads.

A sent e-mail also takes up storage space if it is kept in the "sent" directory.

By the way, I don't think that emails with photo attachments are the problem for the limited memory, unless we're talking about several hundred to a thousand emails with large image files attached.

Apps and videos are usually the biggest memory hogs on a device. Pictures alone come across large masses.