While up until the 90ies (or so), pocket guns usually were made in correspondingly small calibers like .22lr .25 ACP, .32 ACP or .380 ACP (or .32 SW or 38 SPL for the wheelguns), the real tiny mouse guns even in .22 short.
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. Now there are even guns which could be considered mouse guns dimension-wise while being packed with 9mm, or even .40 S&W and .45 ACP.
I wonder whether it makes much sense packing such an amount of power into such small guns, in terms of controlled firing and muzzle energy. E.g. is a 9mm out of a, say, 2,5inch barrel that much stronger than a .380, taking into consideration that the .380 was originally designed for short barrels while the 9mm must exhaust an enourmous amount of unburned powder, propelling the bullet only a little more...? At the same time, in any case, the 9mm is going to kick a lot harder, not to speak of .40 and .45...
I'm sure you firearms wizards on this forum will enlighten me