I can't see ever needing 500 arrows in a stack with even an 18 slot quiver. I can only shoot so fast, and I make no effort whatsoever to conserve ammunition. Because our guild's MT is a very well geared Pally tank, I can even weave Multi-Shot into my normal rotation from time to time and I still only go through 10-12 stacks of arrows tops from Attumen -> chess. Ya, if they upped the stack size by 250% and kept the price the same as a stack of 200 arrows/bullets, I'd love it. 20 slot ammo pouches/quivers on my realm are not very hot commodities, and I don't recall ever having seen a 24 stack pouch/quiver for sale on auction. Maybe Blizzard has decided that the mats required for these things make them highly inaccessible to all but the most hardcore raiding Hunters and is trying to find a way to give the rest of us a break? Will have to wait and see...
(Assuming there's anything to the rumor at all :P)