If you have smaller, compressed images on your PC, they will actually take up more space on the iPad. iTunes creates a few versions on your pictures when it synchronises which is why on first sync with the new photos, you will see the message 'optimising pictures' for a while. I think it generates 3 images, one thumbnail, one full screen and another over sized image that has been optimised for zooming.
I'm not sure how large, full resolution images are handled i.e. if they are downsized and actually use up less space.