I bought in the distance past .22's and .25's for pocket carry. Bought a Kel-Tec .32 when they were first introduced then traded up to the Kel-Tec "P3AT" (.380) when they first came out. I don't own any of them anymore.
My smallest gun these days in both size and caliber is a j-frame .38. For me, it wasn't the tales of mouse guns being unreliable (mine were fine) and it wasn't that they couldn't be shot accurately (paper plates at 7 yards: check). For me it was simple ballistics. I'm no expert, not claiming to be, and never witnessed an actual shooting or even seen a gunshot wound in person - hope not to from any angle - I just feel more comfortable with a treasury load or something similar than anything that can be achieved in anything .380 or smaller.
Even a BB gun is threatening when presented in an offensive manner, but if I ever need to use my carry gun I will need it to STOP some type of offensive attack, not scare someone into giving up their wallet, and prefer a bigger, heavier bullet to attempt that.
I don't try to find fault with anyone's choice for a carry gun, and I do believe that "the small one you have on you is better than the big one you left home". Also nothing of decent power is more easily carried than a KT .380 in my experience. I just decided for myself that my bottom is the .38 and I do what I need to do to have it with me. It's not really that much heavier for pocket carry (although admittedly larger). For me it's a j-frame in my pocket or an auto IWB.
Above is all just my opinion and worth exactly what you paid for it. (Maybe not even that much.)