Why is memory speed slower when they put more on graphics cards?

Like the Powercolor RADEON 9600Pro 128M runs at 300MHz DDR speed, and 256M runs at 200MHz.





The Asus EAX1600Pro 256M runs at 760MHz and the 512M at 540MHz.

Why is memory speed slower when they put more on graphics cards?
From the sounds of it, it could be that the two cards use different processors. As Processor speed increases, it doesn't necessairily mean it needs a faster bus to transfer data through. There's just different technology behind it.