Originally Posted by pcfoo
Most of Kepler cards use 256MB 64bit chips.
The GK110 has a 384bit bus = gets supported with chips in sets of 6. 6x 64bit in parallel = 384bit and 6 x 256MB = 3072/3GB.
Thus GTX 780s have 3GBs (no RAM chips in the back, yes, backplates are for looks only) and Titans have 6GBs (12 chips total).
7970s play the game with sets of 6x 64bit chips, and so where the flagship Fermi cards...1.5-3GB configurations are "natural" for that, and the boards of the GTX 580s had actually room for 24 chips - probably as it was sharing a board with the 6GB Quadro 6000 cards.
256bit cards play it with sets of 4 ram chips, 192bit cards should be playing with sets of 3x.
Those 192bit 6xx cards, are more of a gimmick to "gimp" the performance and artificially create an intermediate tier the low end 128bit cards and the full-blown GK104s.
For 1 and 2GBs in low-to-mid range GTX 6xx cards, they are using an even number of ram-chips that don't "comply" with the badwidth that would require 3x chips to be saturated. i.e,. 192bit bus naturally lends itself to 768MB and 1.5GB cards, but I am guessing that since the natural competition from AMD was 256bit and marketed with 2GB versions, they had to at least match that, even if it meant they would be under-utilizing the chip's and ramdac's bandwidth by limiting the physical electrical connections (or something?).
Selling/marketing of VRam buffer size is much easier than bandwidth.