Scaling and file size

ok, because of time constraints I have to make a doulble of a site that is larger than the original. Fine for all vector images etc. but any jpg images that are imported get slightly pixelated because they are now at about 135% rather than their original 100%. I know that this is bad practice especially since it will be for a real nice touch screen display, but the question is, is there a way to fix it without changing the original image, and more importantly is it going to increase my file size. I am using alpha tweens between images and this slows down immensly after the images have been rescaled.
Anyone know??
thanks guys/girls
erok