What is "(do not include files)" when creating a backup manually?

I’ve just triggered a manual backup with “Yes (do not include files)” and it’s ~20x as small as the automatic backup that occurred yesterday:

I’ve just migrated the forum from MyBB and there are no attachments (the migration doesn’t support them). What causes the difference in size? Is the last backup incremental? Are linked images from remote URLs downloaded in the first one?