Hello Peter,
Like you, I have an > 40-year history with MUMPS dating back before DSM-11.
Not every implementation of the global storage was effíent (no names, no finger-pointing)
What you see today in IRIS is the result of continuous effort on improving performance
and storage efficiency.
In past, the size of a physical disk was the limit. Today with virtual storage systems the
limit is set only by the file system used. This is a fact that InterSystems lives with since "ever". 
A word concerning global efficiency:
Multiple experiences over years showed me that the space consumption with IRIS and 
its predecessor Caché was always significantly lower than with the various source  DBs
I was invited to migrate.  With Sybase, it was rather more impressive than with others.
The immediate consequence of this high efficient storage technology is the gain in speed.
Also with the most modern technologies, there is still a transfer between persistent 
storage  (aka. disk) and memory.  And the less you have to transfer the faster you are.
 
Finally, welcome to this society!
Robert