What is the changed file size in plone during version control?
If you are referring to the Working-copy-support option: Files are excluded from the possibility to be checked-out. Probably exactly because of the reasons you are rising. Versioning attached files is easily blasting the DB because every file would be kept in the DB and thus (to answer your question) the object's size would be a sum of all of them.
Also the standard-versioning-history is not applied to files.
I have a file uploaded to plone say size 1 MB. After checkout and downloading it, I edit and then upload and check-in. What is the size of the new uploaded file in plone site. It is the original size + added size  OR the size of the new uploaded file?