Gel electrophoresis, distance questionn?

I've been taught that the DNA in the smallest sized pieces will move furthest, (and these are cut up by restriction endonucleases). The reason they move is because of the phosphate groups on the nucleotides which are negative, and they move across the jelly towards the positive anode. This all makes sense, but why, in terms of using DNA for testing crime suspects DNA, is it that the one which moves the furthest the guilty one?