At this point, based upon my results, I have to say that my presumption appears to have been 100% wrong.
I created two versions of the "both" (32 & 64 bit combined) Windows 10 DVD ISO today, one with the 64-bit Media Creation Tool that I downloaded on 10/2/2015, and the other with the Media Creation Tool that I downloaded from the website this morning. I've got one more being downloaded as I'm typing from the 32-bit MCT that I snagged on 11/13/2015.
ISO from 64-bit MCT from October 2015: File size - 6,067,904 Kb
ISO from MCT downloaded today, 2/9/2016: File size - 5,750,400 Kb
Both MCT runs completed successfully with no errors, and there is no logical reason for that huge a difference in ISO size except that the source material used to generate both "both" ISOs are different.
I'll report on the final ISO once its generation is complete a few hours from now.
I know I'll always be downloading the latest MCT itself before using the tool to create an ISO if I need to do so in the future.
Brian AKA Bri the Tech Guy (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall