It's a BUG, no it's full sized, no it's a...
Ironically I was thinking of this earlier even before the impromptu XD9SC acquistion, and I was realizing there is a huge segment of the gun market of products that blur the line between a primary and a BUG.
These guns are too large to consider true pocket pistols for the most part, but they are not full sized either.
XD subcompacts, Glock Subcompacts
Kel Tec P11, imho
Taurus Millenium Pro
Ruger SP101 with 3" barrel
S&W K frame with 2.5"-3" barrel
These guns are a noticeable step up size wise from your snubby J frames with 2" or shorter barrels, your tiny .380s, and your NAA revolvers, but no one really considers them true mouse guns.
This size category offers a lot of advantages. For one, most people who decry small guns often criticize capacity and power. Well the XD9SC holds 11 rounds of 9mm... you know what, that doesn't suck! As far as I'm concerned that's more than adequate capacity. The SP101 handles powerful .357 Magnum loads just fine, and honestly five rounds is enough to ruin some gangbanger's day, not to mention more interesting possibilities like loading it with hardcast 158 grain loads for a very small gun that could take down a very large animal.
You also typically get full sized or at least larger sized controls, you avoid the issues tiny parts present, and you get a meaningful sight radius. Plus, I think bigger = better in terms of controllability, which leads to better shot placement.
And you sure have to admit they're easier to carry.
The downside as I see it, is that it's really not that much more trouble to carry a full sized gun that does the same thing but better. You have to invest in good holsters to really make the most of these smaller guns anyway, and by the time you do, you could have bought a rig that would enable you to carry a superior tool.
But still, people's bodies are different shapes, and in my experience smaller people with smaller hands naturally prefer the compact to the full sized, the K frame to the L frame, etc.
I believe this portion of the market will continue to grow, as it has already, exponentially. But due to their size, I think they blur the line between what's considered a "fighting" handgun (oxymoron, I know) and a "backup" piece.