I have no idea to where you are measuring the water capacity of the two cases but it does not really reflect how much powder you can put in a 260 in a short action vs a 6.5x55 in a long action.
That pesky little thing called OAL will get you every time.
Back when I had a SA 260, a 140 took a lot of powder space. In a properly throated 6.5x55 in a LA, that 140 would allow a lot more than a 2 gr difference as it would only take 2/3s of the neck.
BTW the 6.5x55 holds 58 gr of water. http://en.wikipedia.org/wiki/6.5x55mm
Insofar as 1000 yard shooting goes, the 6.5x284 not the 260 rules the roost.

Newt-Condi 2012