@jdaviescoates said in Nextcloud says 5.3GB used, Cloudron says Nextcloud is 19.16GB, why?:
That's "just" a difference of 2.66GB (which I guess could mostly be just Nextcloud files themselves)
So, assuming that is mostly just Nextcloud files themselves (I may do a clean install at some point just to see how big that is), I think this can be marked as solved.