Fullboar,
I think you are talking about absolute case capacity, in which case you are absolutely correct. But in terms of functional case capacity it is a different story altogether.
Taking a look at max loads from Hogdon's data paints an interesting picture.
260 Rem
142 GR. SIE HPBT Hodgdon Hybrid 100V .264" 2.780" 38.0 2464 46,800 PSI 42.0C 2690 57,500 PSI
6.5 Creedmoor
142 GR. SIE HPBT Hodgdon Hybrid 100V .264" 2.780" 39.0 2581 50,200 PSI 41.5 2737 58,900 PSI
You are looking at half a grain less powder for the Creedmoor. True that H100V is probably not the best powder for the 260 Rem, but if we look at the old standby for target shooters Varget...
142 GR. SIE HPBT Hodgdon Varget .264" 2.780" 35.0 2541 54,000 PSI 37.5 2645 59,100 PSI
6.5 Creedmoor
142 GR. SIE HPBT Hodgdon Varget .264" 2.780" 34.5 2489 52,600 PSI 36.3 2598 60,100 PSI
So 37.5 with the 260 Rem to 36.3 with the 6.5 Creedmoor. With most powders there will be less than two grains between the two as long as you are comparing the same powder, bullet, and primer. If you are loading for max loads all the time then the 260 Rem becomes second fiddle to something like the 6.5x284 or 264 Win Mag. But if you are loading for accuracy then velocity doesn't matter as long as it has enough to get to the target without going subsonic (or if it does go subsonic the bullet handles the transition well).
It is only when you have a bulky slow powder that you can take advantage of the 260 Rem's slightly larger case capacity. H4350 is a good example, you get 2.8 grains of spread on max charge and 3.2 grains with Win760.
260 Rem
142 GR. SIE HPBT Hodgdon H4350 .264" 2.780" 41.5 2590 50,100 PSI 44.5 2735 58,000 PSI
6.5 Creedmoor
142 GR. SIE HPBT IMR IMR 4350 .264" 2.780" 39.2 2511 48,300 PSI 41.7C 2687 58,600 PSI
Or Win760
260 Rem
142 GR. SIE HPBT Winchester 760 .264" 2.780" 41.0 2588 50,400 PSI 43.5 2734 58,800 PSI
6.5 Creedmoor
142 GR. SIE HPBT Winchester 760 .264" 2.780" 36.5 2470 50,900 PSI 40.2 2634 58,300 PSI
But, if you want to hit that same 2734 fps that the 260 Rem has with Win760, all you need to do is load the Creedmoor with Hybrid100V.
Jimro