Unlike the other 2, Gentoo doesn't give you binaries. They give you source code (much, much, much smaller download) and you have to compile it yourself (much, much, much longer process).
You save a gig or two in downloads and CD burning, at the expense of spending 3 days compiling everything into binaries.
On the other hand, by compiling them on your own machine, they could be optimized for your specific hardware, which can result in smaller/faster binaries, once all is said and done.
To compare it to hardware: imagine company X sells laptop blueprints. You look and see that the blueprints weight about 4 ounces, while Sager laptops weigh 12 pounds. Does that mean company X sells lighter laptops? No, you still have to follow the blueprints, which means you'll wind up building a 12 pound laptop, and it'll take you a year and a half.
But, you're cool, because you built your own laptop