Back when I started carrying in the mid-1990s if one wanted a "pocket" gun the two basic choices were small .380s like the Colt Mustang (which wasn't ideal being single-action), the Walther PPK, and similar sized guns or J-frame .38 Special revolvers. None of the small .380s were anything even close to the reliability of a J-frame. Then there was cartridge effectiveness. In terms of "power" the .380 ACP and .38 Special are pretty close. However, the .380 is marginal at best when it comes to getting both sufficient penetration and expansion. The .38 Special is much better at getting both penetration and expansion and also has a much wider variety of ammunition available to it.
Back when I started carrying my go-to pocket gun was a S&W 442 in nickel loaded with standard pressure 125 grain Federal Nyclad HPs. Interestingly enough, that hasn't really changed. Unfortunately I don't have that particular 442 anymore but my usual pocket gun now is a S&W 442-1 loaded with Buffalo Bore 158 grain LSWC standard pressure ammunition. My spare ammunition is either Speer Gold Dot 135 grain +P or Winchester Ranger 130 grain +P. I use that ammunition for reloads rather than the Buffalo Bore because the lead bullets are extremely soft and deform easily outside the chambers. Additionally, the profile of the JHP ammo is much easier to load into the chambers quickly. Well, quickly for a revolver reload with speed strips.
That said, and as much as I like small revolvers, I think the most size efficient guns on the market today are the "Micro 9s" like the Springfield Hellcat, SIG P365, Ruger MAX-9, etc. They hold twice as much (or more) ammunition than a .38 Special and can generate enough "power" to both penetrate and expand sufficiently. They also tend to have been sights, better triggers, and better ergonomics than small revolvers.
Here are a couple of examples:
Ruger LCR .38 Special compared to a Springfield Hellcat 9mm. The Springfield holds 12 rounds in this configuration and is smaller than the LCR (the LCR with the stock compact Hogue grip is about the same size as the Hellcat):
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Here is a 3" .357 LCR vs. a Hellcat RDP. The Hellcat is smaller despite holding 14 rounds and having both a comp and a red dot. Granted the LCR is a .357 but it is basically the same size as the .38 Special 3" LCR.
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The .38 Special still has the advantage in ammunition versatility, especially for handloaders. I have some .38 Special I handloaded in my safe room that has 200 grain LSWC bullets. Try finding a 200 grain 9mm and making it function.
In general I think the small .38 Special revolver (or these days .327 Federal, .357 Magnum, or 9x19mm revolver) still has major advantages for pocket carry. Their profile is better suited to drawing from a pocket, they are more tolerant of things like lint, and they tend to be slightly lighter in weight. I still carry revolvers when I pocket carry but for overall efficiency I think the 9mm pistols win.