800x480 = 384000 Pixels
1024x600 = 614400 Pixels
1280x720 = 972800 Pixels.

By jumping from the current screen size to 1024x600 you almost double the amount of pixels per screen refresh. If you jump to 1280 it is almost 3 times the amount of pixels. This would mean the processor/graphics system would have to handle a LOT more data per-refresh.

Nathan