As a general rule, the smaller an auto loading pistol becomes, the more finicky the mechanical operation. Spring tensions and slide mass all work together to make an auto loader operate dependably. Small guns are harder to refine to reliability. They also typically exhibit far shorter longevity. For these reasons I prefer the small frame revolver for deep concealment needs. However, for those who prefer the small auto, and there are certainly some advantages to them, how do we asses reliability?
First, let’s define what small actually is. If we look at compact auto loaders, which are just reduced size versions of the full-size pistol, these typically perform well if the overall family of guns is reliable. Guns such as the Glock 26, M&P Compact, or carry versions of the Sig p320, usually give up little, if any, reliability to the full-size version. This size pistol is my preference for carry due to the fact that they are easier to carry than the full-size variant, yet perform at a close level, especially if compatible with the full-size magazines. I find such guns to be weapons that are concealable in most clothing, but also fully capable fighting pistols.
Probably the single most popular class of handguns for concealed carry today are single-stack 9mm autos. Some of these guns are actually very solid performers. The top two, in my opinion, are the Smith and Wesson Shield and the Glock 43. Both prove reliable and generally do not pose issues. Others in this category, however, are plagued with problems. The variants of this class that trend to the small side can introduce significant reliability issues. Feeding reliability can be problematic with some of these weapons. My absolute recommendation for a single-stack 9mm is the M&P Shield. The Shield is solid, but it is actually on the larger end of the single-stack category, and that is perhaps no coincidence. Guns chambered in 9mm can only get so small.
Another issue that comes into play with small single-stack guns is, quite simply, the smallness itself. It becomes difficult to adequately grip these little guns and work the controls. These complications in grip often lead to limp-wristing, thus inducing failures to feed, and a consistent trend I see with small autos is the inadvertent dropping of the magazine. Most often when I see someone compete at an IDPA match with one of these tiny 9mm pistols there are failures. This is significant because the pressure of a match usually introduces complications to shooting that most people do not experience when practicing by themselves on the flat range. Small autos are potentially problematic and I really suggest that you use yours for competition matches and take a class with it to ensure that it is reliable equipment.
For these reasons I personally prefer the small revolver when I need a deep concealment gun. My smallest auto loader is my Glock 26, which has proven every bit as reliable as the larger guns within the Glock family, and I can shoot it and manipulate it almost as well as the full size variants. If I step down to single stack guns I cannot achieve the same. The afore mentioned M&P Shield is a very good shooting gun for its size, but I find there to be few circumstances where I can conceal it yet not be able to conceal the Glock 26, which is a much more capable defensive tool that can use full-size double-stack magazines. When I need truly small, I go with a snubby revolver and call it a day as I find the small-frame revolver to conceal even better than single-stack 9mms due to the round and organic shape.
The class of auto loader that conceals even easier than the small revolver is the tiny pocket pistol, most often chambered in 380 acp. These guns serve a purpose, as they can be carried in almost any clothing, but they are inherently challenged in reliability. These guns are notoriously finicky with ammunition selection and if you use one you must truly test it with the ammo that you carry. I personally do not trust any of them. Many who claim that they have one that is reliable never test it realistically, such as pulling it out of a pocket after being carried for two weeks and firing it with a poor single-handed grip. A perfect two-handed grip with a clean gun on the range is not a realistic test for such a pocket gun. If you have a pocket pistol, test it realistically.
Your mileage may vary, but if you use a small auto I encourage you to truly put it to the test to ensure reliability.