Looking through the (Firefox) devtools, there is only one cookie (being my login token), but a fair bit of cached images (and IndexedDB data for managing said cache), which is what I assume most of that size is.
I wonder if it is related to how the browser is reading the websites. With the new version, I noticed I now have scripts from every instance that is 'touched' in any way running, rather than just the instance I'm using and viewing the others through.