As a rendering engine we allow displaying of our rendered image in the viewport. Because SUOverlay API expects buffers to match the viewport size 1 to 1 we have to upscale these images in a second buffer. This is very slow when compared to the speed that can be achieved on the GPU.
Unless there is some technical reason of which I am unaware that would get in the way, it would seem to make sense to have SketchUp upscale/downscale the overlay buffer to fit the screen.