If you render a bump map, the graphics system actually creates geometry and draws it. This is a normal map, which just fiddles with the lighting to make it appr

If you render a bump map, the graphics system actually creates geometry and draws it. This is a normal map, which just fiddles with the lighting to make it approximately match what you would get if the details were actually there. The normal map probably takes up slightly more memory than all the geometry that is in the more complex model, but the more complex model causes the gpu to render a lot more triangles which is a lot more work. Looking up values in the normal map is something the gpu is very good at.