Installing minimum version

The official ISO from the downloads section is the minimum space and effort to get it going.
There are ways of getting smaller footprint, but that requires quite some effort and working from a terminal, starting with using a net ISO of Debian and adding things as needed.

The ISO contains LinuxCNC and the required RT kernel.
RTOS is a different project.
And yes, it is possible to remove stuff you do not need, but i doubt it will save much as it already comes with litle extra stuff.
What is the issue with space?
Storage is cheap now, so...

Motion control software, in this case LinuxCNC is very small, about 15MB in size.
The problem is that motion control software needs an underlying operating system so it can function, and that takes a lot of space.
Having said that, i wonder why, windows 10 takes over 60GB of space while almost all Linux distros i use take under 8GB of space, Mint 21 with plenty of other software added takes 13GB.

I am sure if you put some work in you can make it under 1Gb.
Have a look at
forum.puppylinux.com/viewtopic.php?t=5213
Or use this and add the realtime kernel and LinuxCNC, should be easy enough as those are in the debian repositories already.
puppex.exton.net/

Almost forgot, you can always use TurboCNC that runs under DOS, so total space would be about 1 MB.
That is 1 megabyte.