Crossbreed sells the SuperTuck Deluxe for fullsize and compact M&P's (the same is true of other pistols like Glock and Springfield XD's). They advertise that you can use either holster for either FS or C.

So what is the difference you ask? I emailed Craig and got the answer, but it is a little easier to see than explain.

Here is my M&P fullsize 9mm in the fullsize holster:

Notice how the pistol comes to the end of the holster.

And now my M&P compact 9mm:

Notice how there is a little extra leather past the muzzle.

And there you have it!

Obviously, I chose the FS holster (since you are looking at the holster in the pics). The little overhang of leather is not noticeable. If you get the compact holster and have a fullsize obviously the pistol will hang out of the holster slightly.