Oh dear base64 encoded images in the main content.
that also happened when you copy /paste content with images from Word into a Tinymce widget.
Iirc @mauritsvanrees wrote a proof of concept some years ago (or longer) which extracts those images into separate Image content items on save. Classic UI. Let me search.
Found it, 4 years ago: GitHub - collective/collective.base64imagepatch: Assuree that no inline base64 encoded image is stored in an RichtTextField
Maybe suitable to grab some code from 