Nvidia I believe is running the biggest GPU chassis as the classified versions are fat. Most cases can't even hold them. As far as AMD I don't think "BIG GPU" is their marketing ploy just that with the current generation their PCB's require so much room since the components have not yet been scaled down. So the only way to make a GPU more powerful requires them to use a bigger PCB to hold it all. On top of that come bigger Cooling units... AMD is no known right now for it's quality coolers seeing as though their Hawaii GPU technlogy is the hottest running GPU on the consumer market and in return they create a cooler that is louder than a hair dryer and cools things down as much as throwing it into the sun.