Enlarging rimfires also runs into a strength of materials problem. The larger the chamber (and hence the cartridge), the thicker the material needed to contain the same pressures.
To illustrate, assuming a .22LR cartridge develops, say, 20,000 psi, the thin brass can hold that in a "vessel" the size of a 22 LR case.
But put that same pressure in something the size of a .380 cartridge with the same wall thickness and it will rupture. The tensile strength required to hold it dictates a much thicker vessel wall.
Force = pressure times area.
Since the outward pressure is vectored into stretching (tensioning) the case wall, it can be seen that the tensile forces on the wall are sort of multiplied by the internal area, that is to say, the diameter of the case. The bigger the case, the greater the area the pressure has to work on. In turn, then, this results in greater tensile forces on the cartridge case wall.
A simple analogy:
It's like blowing up a party balloon. You've surely noticed how hard it is to expand it at first, but as its volume (and therefore its internal area) increases, it gets easier, at least until it's ready to pop.
While it is true that the chamber supports the cartridge, in any firearm, there are inevitably areas of the case that are unsupported. So scale that same principle down to these unsupported areas, and you have the same tensile strength problems, albeit on a smaller area..
Imagine a rimfire 5" artillery shell. It could not be made out of brass the thickness of a .22 case. (Apart from melting and handling problems.) Unless it were absolutely 100% supported in the chamber, it would rupture at any small unsupported area.
I always wondered about the new "Short Magnum" cases, where they retain the same internal powder volume, but with a bigger diameter case to make up for the shortening. The tensile forces on those bigger diameter cases (and therefore the chamber walls) must be enormously increased, compared to a conventional long case of the same volume, where the diameter is smaller.
Terry, 230RN