Thanks to all for the answer. I'm assuming the larger x number, the longer the
case and the longer the case the greater the load of powder?

True to a good first-order approximation: case capacity matters a
great deal, and certainly gives you a decent expectation of the
performance class to expect.

In a broad view, the full-power thirtysomethings (.303 British,
7.62x54 Russian, 8mm Mauser, and .30-06 and its shorter, somewhat
differently shaped successor 7.62x51) did about the same thing in
about the same way; and that is rather more (at a penalty in weight
and volume and recoil) than the lighter cartridges that have been the
postwar trend, and far more than you would get from a reasonable
sidearm-type pistol; and they look as though this ought to be the
case (no pun intended and not much of one achieved).

Of course, God is in the details and so is the devil. Length doesn't
tell the whole story; the shape/diameter of the case, the technical
limits and conventions restricting the pressure of the cartridge, and
the choice of powder type and bullet weight can all be substantial
variables. The result of all this is not only differences between one
cartridge and another, but a range -- sometimes big -- of performance
potential for each cartridge.

Usually the military chooses one or a few points in this parameter
space and sticks to 'em. They are not necessarily maximal -- the
way that bolt-gun-only hunting loads for .30-06 can bend certain parts
of the M1 Garand is a case in point.

Shape is interesting. Makers of civilian rifles and ammo have been
exploring the power and accuracy potential of shorter, fatter
cartridges. Of course, their customers' priorities are much
different than the army's, including many factors that bear upon the
number of rounds in the magazine; and there is more variety in what
they need. Individual hunters and target shooters also have a lot
of freedom to chase the latest trend in search of better performance
(and/or treat themselves to a new bit of sporting goods) -- polar
opposite to the military's motivation to buy, maintain, and train upon
huge quantities of a few standard items.

So one hardly expects, say, .243 Winchester Super-Short Magnum to be
the next military cartridge; my guess is that, role by role, the
continued resurgence of 7.62x51 amid the existing assortment is more
likely, at least until something entirely different comes along.
But it goes to show how complicated the parameter space is, and how
much innovation (some of which, admittedly, will always lead up blind
alleys) is still occurring in what one might have imagined to be a
very mature field.

Cheers,
--Joe