I must be cranky today, because I am going to talk about nomenclature, and what is the point of that besides arguing?
Anyway, traditionally, the Colt 1903 and Walther PP and FN 1910 and Beretta 1934 were the size considered to be "pocket pistols". They may have meant jacket pockets (men wore suit jackets and topcoats a lot more back then) or maybe trouser pockets were a lot bigger because people were not trying to wear form-fitting clothing. This was a term generally used for automatics; comparable revolvers were called "belly guns", which was a vaguer term, at least to me. Smaller guns were called "vest-pocket" pistols and were typically 25 automatics. There was not a term for similar sized revolvers that I know of, even though little top break 32 S&W revolvers (also made in 22 rimfire) were very common then. The Seecamp, the first of the new generation of vest-pocket sized 32's, was actually originally introduced as a 25.
The definitions have changed somewhat if guns like the Kel-Tec 32/380 or the Ruger LCP are now pocket pistols, a change that makes sense given that pocket sizes have changed. 9mm has taken over the OLD pocket pistol size, and the old vest pocket size 25's have largely died out, because while 25 ACP makes a good threat, it is less good when it comes to actually getting things done.
So expect some confusion when using the term "pocket pistol". Millions of old-size 32's and 380's still exist, and while the new generation of small 32's and 380's are still called that, they are quite different. It might have been good to have a new name, but "pocket pistol" fits them so well that that isn't going to happen.