Scaling is slow.
Well, I remember that in slower systems, it was usualy better to split and image into smaller images.
I think drawing a lot of small images is slower than drawing one big image, because of the extra work that has to be done for each individual image (as the total amount of pixels is the same). When I tried my tile engine with smaller tiles it got slower, if I remember correctly. But maybe it also depends on what kind of video system you use, how much memory it has, etc.