Pretend for a minute that you had never heard of the M&P line before ...
This line of guns has a full-size model that uses 17-round magazines. A couple of compact models use 15-round magazine. What common gun term might be used to describe a model that uses 12-round magazines?
When S&W filled the space between the two 1.0 sizes with two new models, sensible names became a bit scarce.
The issue is when you compare across different brands. This is nothing new in that there is no standard naming convention around "Compact", Subcompact", even "Fullsize".
So let's go with round count.
My M&P with a 4.25" barrel and 17 rounds would be a fullsize, but my Beretta M9 5" barrel with 15 rounds would be compact.
How would you classify "single stack" handguns?
A Gov't 1911 take 7/8 rounds, so does the Shield.