I've made a similar move myself from Evernote and its clipper bringing around 13k notes at just under 5gb, and strongly suspect a good portion of your data must be images.
If you have an elaborate tagging system you may be able to search them efficiently, but I'd encourage you to consider whether you really need to keep them in a note taking platform.
If not I suggest 2 options. Either all export notes to markdown and re-import into a new profile retaining clean text notes, or second try to clean up your attachments and delete noisy stuff, in which case this may be useful:
I am personally holding off on this tool until I see a little more feedback, but if you have not yet made a commitment to joplin you have nothing to loose! Please share any feedback.
Good luck