+1 Have a 9mm and the size is GREAT! Not to mention the Shield trigger is better than ANY striker fired sub compact I've tried. Lined up a Ruger, Beretta Nano, Kahr and Kel-tec the day I bought the Shield.
I'd definitely check out the Kahr. Mine is a CM9 (9mm), but the .40 version is about the same size. I looked around very hard before buying it. It has no safety, a plus for me in a defensive gun, a good trigger (it is a long, "soft" trigger as opposed to a crisp, clicky one, a factor some may not prefer but I like it). It is smaller and lighter than some other popular offerings (and has less "issues").
I'm not sure how small you are thinking but my CZ-P07 does not seem to be too big or bulky and performs great. The 12+1 capacity is a bit larger than the others listed above, but its a great gun.
I have a Sig238 that I carry in the warmer months bit I am wanting something larger in caliber but still small for the winter months. I like the Kahr CM40 and I've had the Glock 27 in the past. I am a small frame guy and it can be difficult to keep it hidden if any larger.
The Walther PPS for sure. I loved mine except that it didn't fit my hand very well (full size Glocks fit great), I loved how easy it was to conceal and I loved shooting it. I tried a Hogue wrap around grip on on it it improved my shooting greatly but at the cost of the slimness that was part of what I loved about it. I have it for sale primarily because I am no longer shooting .40.
I understand you said single stack. But depending on how you want to carry it, I'd consider the glock G27. I used to think it was large, but over time, it carries excellent and very comfortably. My buddy has one of the Walther PPS. Nice shooter too, but the G27 is my pick...
I've carried the Taurus PT709 for 4 years and its been reliable....I know you said 40 cal. but the PT740 is the same, just caliber difference.
Some dont like Taurus, Ive been "lucky" three times.
For a single stack 40, I don't think it gets much better than the Kahr PM or CM series. I have my CM40 on my belt as I type this. Great trigger, reliable, and very concealable. One of my favorite carry guns.
I've thinned my handgun collection down to primarily 9mm and .45ACP because I've never found a .40 that I can shoot as well on the same platforms with one exception...the Springfield Armory EMP.