Configuring Office 365 ProPlus updates for remote workers using VPN

For Office, updates are always a delta, it never pulls down the full source files to update the product. There are basically two types of delta: binary delta and regular delta. Update sizes are documented here: https://docs.microsoft.com/en-us/officeupdates/download-sizes-microsoft365-apps-updates

If Office updates to version N and the installed client is on N-1, it will download only the actual changes per file. E.g. 1K changed in excel.exe, it will download just this 1 K instead of the (compressed) 55MB excel.exe. This is called "binary delta". As mentioned, this is leveraged when updates are only one release apart OR when a new SAEC-Preview comes along (e.g. from 2002 to 2008) or a new SAEC (e.g. from 1902 to 1908) for the first time.

If installed version and targeted version are not in one of the above mentioned relations to each other (e.g. updating a machine which was offline for a while from Current Channel 2001 to Current Channel 2004), it will download each changed file (e.g. the compressed(!) excel.exe) and stage this on disk.

Note: When assessing the update size, always measure what is actually going over the wire. Looking at files on disc is misleading, as these files are already decompressed, sometimes sparse files and merged from downloaded bits and files already on disk.