If anyone is curious, here are pictures of the M&P M2.0 3.6" Subcompact's three size Configurations.
The grip sleeve for the compact (15+1 round) configuration is a bit loose. The sleeve for the full-size (17+1) magazine, however, is nice and tight comparatively.
As you're probably aware, when you get the Subcompact it will come with the two sleeves (one compact and one full-size). You might just buy both from the outset, or you might be looking to choose one one to start with since adding magazines can get pricey. If you don't already have a preference, I recommend the full-size just because it makes the gun a little more controllable due to its larger size & shape, the grip sleeve fit is better (tighter), and there are more more rounds if you carry an extra magazine (without adding much length in the magazine).
Of course, if you think for concealment purposes you want to start out with the compact, by all means do that. And like I said, if money isn't an issue, buy both. If I didn't already have other compact carry options and I had to decide one over the other, I might opt for the 15+1, but I do, and I like the full-size better for the reasons I stated herein, but everyone is different.