Without too much effort you can bring a SF DVD image down to 2/5-3/5 of original size with Windows compression (there are many identical dummy files on SF media anyway), but not to some MB or even KB; and such a image won't start everytime. The reason for this is that ISO check is changing all the time.
Look at
THIS thread; with some Bushound logs you may find a big gap (in the first 1/4) which sectors are almost never included in ISO check. Such image will be authenticated most times.