I feel there should be some simple way to show the sizes, in some List view.
If you use iCloud to store them, and iCloud builds up, by that very nature, there really has to be a way to view what's taking up the space, at a glance.
If you have over 7,000 photos, going through each one is not right. It should be sortable, and to view it by that amount.
I accept images have multiples, the way CMS does to have a Thumbnail version - BUT, the image in question is the one that I suspect is what the system uses to state what size KBs or MBs you are taking up.
Either way - the same way a system like WordPress does, it has a default size. Delete that image, and it deletes the relative thumbnails.
This should be possible, in the iOS system itself.
Whether it is on a FileManager or Image manager - they are all technically files.
Hopefully there is a free way to do that.