Confusion about Microsoft Exchange 2013 RTM .ISO and Cumulative Updates

I don't know if it is "well-known".

I agree that the documentation of this feature is poor. Here is some Microsoft Documentation:

Updates for Exchange 2013

Exchange 2013 cumulative updates and service packs include all the changes from previous updates and are full builds of the product. You don't have to install the previous cumulative updates or service packs to get the latest features.

Servicing Exchange 2013

Each quarterly CU package will be released as a full refresh of the Exchange product and will be installed as a build to build upgrade

The purpose of the DVD .ISO is for companies deciding not to update to the latest CU/SP. Same as you can download each previous CU/SP of any Microsoft Product.

I found out the difference in size
the .exe is compressed
it's 2.8 GB once you uncompressed it
it is missing some Unified Messaging language packs (450 MB) as well as a "Search" folder with strange data in it. Seems to be a FAST Search Engine (MS Product) - don't ask me why it's on the Exchange Product DVD though :)