A 3D game has to render polygons, their textures, detect collisions etc. and current GPU's eat an enormous amount of power. Which aspect(s) eat the majority of the power used on a GPU when a typical modern 3D game is being run? A second question: At the low-level, what are the most common instructions of the ISA being executed when a typical video game is being run?
Of course, you can always take aspects to the extremes and create an artificial program to increase any one aspect X and then say "X eats the most power". However, I'm referring to central tendency among current 3D games with reasonable quality settings.