A have tried to find a way to actually see what uses up my storage, but it seems there is no simple way to see the storage usage of files at a glance, only if I go one by one and check each and every file? (More weirdness is that Trash needs to be emptied but it don't tell the size even by individual infos.)
If there is such a thing I don't know how to find it (and why it isn't obvious); if there is none then definitely there should since users may run out of quota but they cannot tell why nor can they know what do delete, or optimise (and how, but that's another topic).
