From this link:
SharePoint Online is supposed to support Shredded Storage and, if I'm understanding it correctly, then this is supposed to store only deltas in document versions.
I tested a fairly large document (~10MB) and it looks like full versions are being stored and not just deltas. I am looking at the site collection size (admin portal), document library/document size in Site Settings>Storage Metrics and Version History and they all add up to full versions and not deltas.
Can anyone confirm or verify if only deltas should be saved?