Yesterday, just by accident I came across this 5 pages of noteworthy article which solved this mystery for me & put an end to my never-ending nightmares!
On page 5, you will find this:
Each instruction code, memory address, etc. will take up more room in your RAM and cache due to the increased length of pointers, protected space, etc. Where a 32-bit OS may be able to execute a program in 100MB, you can find that the size of a 64-bit OS running the same program can take 105-110MB. Often times this isn't the case, but it is an important consideration.
Caution: the article starts with the basics that could be offending to technical audience! Viewers desecration is advised!)