I can't speak for all games, but I find my fragment shaders dominate. That is compute power related directly to the number of pixels rather than polygons.

If I can render objects in the distance at 1/3 the spatial resolution, that is 1/9th the fragment shading, times a much reduced texture bandwidth since I'll be using a lower resolution mipmap.

I'll still have to composite it back in to the full resolution frame, but if that operation is faster than my fragment computations it would win.