I am not sure how does sharepoint online managed the versions for the documents inside document libraries? For example let say that i have a document with 50 pages and the document is on version 1, and i edit the document where i change a font color for a single word, and i save back the document, where the document version get updated from 1 to 2. then will sharepoint stores 2 full versions of the document? or it will only store the differences in the second version?
A web search for "SharePoint version shredding" will get you a lot of info. How Microsoft stores data on the back end of SharePoint online is not visible, or configurable, to us. I would assume that Office document shredding works like it does on-prem.
As far as size reporting, all versions report the full size of the document, regardless of how it is stored/shredded in the database.