In recent years, that seemed to have changed, since there are a lot of real small guns in calibers which were formerly confined to full (or at best compact) size service pistols.
My guess and this is only a guess is “weight”. Since the invention of the reliable plastic gun you can carry 9mm or larger than guns like the PPK and have the same weight.
What do you consider the upper end (size) of a pocket gun is?
To me the smallest usable gun is like the Beretta Bobcat but its 25 is so anemic that I wouldn’t carry one for a personal defense gun and it’s too small for my hands.
Again in MO one of the best guns is the Ruger LC9 or LC380.
I have fired them both and I’m sorry I don’t feel that the 9 has that much more recoil. It has more but it’s controllable as any small auto. There are other good 380’s and smaller but the advantage of the small guns has to be weighed against their disadvantage.
Recoil is in the mind of the beholder, a pocket gun is not a target gun and when you need it for self defense you’re not going to reload it several times. You’re just going to point and shoot and adrenalin will overcome any recoil. And since the distances for confrontations by history are short again recoil and accuracy are not the most important. Practice is what’s most important.
I have owned Walther PPk’s Mauser HSc and several others all in 380. This was back in the 80’s and I carried them. But I gave them all up for carry for a slightly larger gun that was easy to carry and had considerably more energy.
Charger Arms Bull dog in 44 special.
If energy is not enough or if recoil is too much for you, then you have the wrong gun and you need to look somewhere else.
"The difference between stupidity and genius is that genius has its limits" Albert Einstine