Lined up two rifles side by side for you if you wanted a size comparison. One's a full length 'M16' with a full stock on it, while the other is a basic TM M4 with a collapsible stock on it. The collapsible stock opens up to the same length as the full fixed stock, but you of course can now select which length your most comfortable with.
Not to mention you can buy Stubby M4's which have a really short front barrel, if you want a good CQB gun thats easy to handle in tight quarters.