Although many people might not notice, but this sort of relationship is unique and one of it's kind, this is the most effective relationship an OS can have on another.
The devs of the binary based distros have to compile packages on their own. They need to take care of it's dependencies, compile time parameters, environment, any sorta group etc... this's what all Ubuntu, OpenSuse, Fedora etc... devlopers have to take care of, except Sabayon.
For Sabayon devs all this job is done by the source based distro Gentoo, this might not sound great, but actually it is. You see both Sabayon and Gentoo have users, so by this relationship the users can utilize the efforts that the devs do while making an ebuild (or Gentoo packages if you don't know); efforts like the dependencies, compile time parameters all can be user set in Gentoo... and this is what the Sabayon devs take advantage of. All they do is simply compile the Gentoo ebuilds with standard parameters so as to make a great generic desktop, thus the Sabayon team is effectively of the size Gentoo + Sabayon devs and the Gentoo users are of the size Gentoo + Sabayon; making a large community.