mounted ISO; What determines available room?

[Harry Putnam]

Making more room in a mounted boot iso.....

** Subject: How to increase the size of root (/) on an ubuntu installer
ISO **

Sorry that this is a close duplicate to a previous post with above
Subject line. It was only a couple of days ago but I managed to forget
having posted here anyway.

I'd still like to know more about how this is done.  I understand from
answers to my previous post it would be done in the code
somehow... but where would one go to find how that would be done?