they render 1000000 images ...
each still sprite is around 50kb ... ussually compresed a bit ...
and if you take 1000000 x 50 ... it's just 50 megs ... that's not much for enough graphics for the whole game, but too much work we 3d'ers do ...
* when you make something from 3d ... you can render it from different views, so you don't acctually work on all the graphics from scratch but on each object from scratch
that's still a lot of work, but i personally like it more than 3d because of the huge quality difference ...
* Like in game I may start there are sprites for quality stuff, but a building for example may be 3d and with good textures so it would look good ... so ussually the basic landscape is made from patterns and textures
hope it helped ...