A 33 grain case capacity is small compared to 54 grain for the 6MM Remington which is a 65,000 PSI cartridge. I suspect that the 6MM ARC like other short magnums will do well with limited projectile weights but fall short compared to larger capacities.
You're confusing pressures and performance. A huge capacity case cartridge may be limited to under 40,000 psi whilst one with a fraction of its physical capacity is rated at 65,000 psi. The determining factor on setting pressure ceilings isn't case capacity rather case and/or firearm construction and strength.
We've long had cartridges with a duality of acceptable pressures. Initially, it was solely down to the cartridges being designed for weak 19th century actions. The classic example is 45-70 Govt with three pressure determined sets of load tables in manuals according to action type - Trapdoor; M1886 lever-action; Ruger No.1 / Siamese Mauser. The 19th century Mausers are similar with SAAMI having set modest pressure ceilings for US loadings to suit M1888/91 and M1893-96 military actions, but where today's handloaders (and European ammunition manufacturers too for that matter) routinely load them to c.60,000 psi pressures for modern rifles. 6.5X55, 7X57mm etc.
In more recent times, the phenomenon hasn't so much reappeared (because it's never gone away) but is becoming increasingly common principally due to people designing an ever wider range of new cartridges for the AR-15 platform and the increasing practice of using three case-head diameters with a standard size AR-15 bolt, with the case-head dia. affecting lug thickness and thrust back on the bolt face. The bigger the case-head the lower the pressure that the AR-15 can take hence the Grendel and ARC with their 0.440" dia. heads being SAAMI limited to MAPs of 50,000 and 52,000 psi respectively. As
@gunsandgunsmithing says though, that's a firearm inspired limitation, nothing to do with brass or case strength and boltguns designed to handle 60,000 or higher pressure cartridges cope equally well with the Grendel size case-head, in fact arguably better compared to the next size up, the Mauser / 30-06 / 308 etc 0.473" dia. design.
What case size does affect is how much energy can be packed in behind the bullet at an acceptable MAP. This in turn directly affects performance as measured by MV and ME, but not in a linear fashion. On performance variations arising from case capacity / charge weight factors, there is a simple and usually very accurate rule of thumb.
If all other factors (barrel length, peak pressure, bullet calibre and weight etc) are equal, then the % change in case capacity sees a % change in MV of a quarter of the capacity change. Using your capacity values, the 6mm Rem's 54gn capacity is 21gn or 63.64% greater than the ARC's 33gn, so at the same pressures will see an ~ 16% increase in ME
all other things being equal. So if the ARC gives a particular 6mm bullet 2,800 fps in a say 24-inch barrel, you'd expect approx 16% higher MVs from the 64% larger 6mm Rem, in this case ~3,250 fps also from a 24-inch barrel. That won't of course apply to the SAAMI ARC loading or handloads for AR-15s as they're firearm limited to a lower pressure.
In terms of ft/lb ME per grain weight of powder that makes the ARC a much more efficient machine than 243 Win, 6mm Rem or Creedmoor and similar as
@gunsandgunsmithing says when loading for boltguns. The '25% rule' only applies too to designs that are reasonably efficient, that is not too small a case to be unable to hold enough powder to achieve peak pressure and at the other end not so over bore capacity that huge charge increments only produce small MV gains.
The growing popularity of the AR-15 platform for civilian range use and hunting aside, what has changed the playing field on this issue is modern propellant technology and the huge range of powder types, burning rates and specific density/energy levels now on offer. A generation or two back the ARC and Grendel would likely have been relatively poor performers simply because the available powders would have seen the case over-full long before peak pressures were achieved. I've no experience of the ARC or Grendel based 6mm wildcats, but a fair to middling bit with its bigger 6.5mm Grendel brother and powder selection is key. There are many suitable grades available though - Viht N133, Ramshot Tac, Accurate-2520, IMR-8208 XBR, Hodgdon CFE-223 and others - that pack enough energy into the little case to achieve good results.
