Accurate shooter published an article with a chart, it is the most detailed explanation of the term I have seen and provides a little more clarity around an ambiguous term. It is also the only way I have ever heard the term discussed but slower burning powder have been available to handloader much longer than I have been alive so if you say someone used the term as you describe I believe you.Afraid not.
It was coined by some gun rag writer and stuck. It has never been used to describe cartridges by a ballistic expert..
Expansion ratio is the official ballistic description of case capacity to bore size.
Cheers.
Also,I though expansion ratio was highly dependent on barrel length as well as case capacity and bore size.