I think that the shape of the hole(s) are also as important as the sizes.
For example, if I theorize way above my pay grade, I would imagine the shape of the distributor advance goes like this... They wanted a very narrow spike of vacuum oat a specific pedal position, so they used a narrow slit. And I can affirm (from driving around with a vacuum gauge T'd into that line) that the vacuum spike is very narrow and very high (vacuum).
The EGR ports on the other hand, appear to be placed to provide vacuum over a much broader pedal position. I'm guessing the smaller hole in the back is used to "extend" the EGR vacuum value deeper into the throttle.
So my read on the whole thing is... They optimized the ignition advance to kick in only at one narrow pedal position (light cruise), but the EGR is activated over a broader range and deeper into the pedal (medium cruise).
But, neither of those ports will produce any significant vacuum once you get above medium cruise.