Saving only small files on my computer

Hi, Any thoughts on this. I shoot in RAW, do some work in Aperture & PS & end up with a large file, 35M or so. If i save that to an external hard drive for prints, i can then go back & reduce the large file to something much smaller, for viewing on the computer. I figure this will save me a lot of storage space on the computer, & also how much reduction could the large files take? Thanks GeoffH