Hi xtra330,
I am not sure exactly what configuration you are talking about. If you use a PowerExpander you would plug the Equalizer (or MatchBox) into the PowerExpander and then plug the servos into the Equalizer (or Matchbox). Yes, you are necking the power down to one wire if you do it this way. Both the MatchBox and Equalizer 3D recommend that you use the auxiliary power input when using more than two servos. In the MatchBox case you still end up with one wire but they way I see they figured it, that wire should be a short wire to a second battery. A short wire has less resistance than a long wire so you get less voltage drop. The Equalizer 3D has two aux power inputs so you would be running four servos off two wires. The other thing to consider is how the power is getting to the Equalizer (or MatchBox). If you use a receiver you have one or two 22ga wires with 3A plugs. This limits the current that can come into the receiver to start with without seeing large voltage drops between the battery and the receiver. Using the PowerExpander you have Deans plugs and at least 18ga wire, most of the time two 18ga wires. The 18ga wire can handle about 4 times the current of the 22ga wire comfortable and the Deans plugs can handle nearly 50A. You get a lot more power (less voltage drop) to the servo connector on the PowerExpander than you do to a servo connector on a receiver.
Please let me know if I need to expand on any of this for you.
Thanks,