More importantly, every configuration option of every package is enabled (I mean the "configure --with --enable" build-time stuff). That's a *lot*, a *LOT* of stuff, making the software bigger and more complex. And they need to, because the software needs to support all the possible things the users want.
To give a specific example, mplayer in Ubuntu needs to support dozens of stuff like (random picks here): directfb, esd, ftp, ipv6, jack, joystick input, lirc, openal, samba, etc, etc, etc, etc. It's huge list of features compiled-in into mplayer.
Gentoo doesn't work like that. I have all of those things disabled; they're not even in the executable. In Ubuntu, being a binary distro, all of that stuff is compiled into mplayer, regardless of whether you use the stuff or not.
Now imagine this removal of "bloat" on a global scale; every program and every library too in your system can be trimmed to only have stuff in it that you actually use. This is "bloat removal" on a level no binary distro will ever be able to provide.