Supply and demand is one thing, but there are also "ceiling" effects. This is just a hunch (and I'd love to hear some feedback on this) but I would guess that there's more of a ceiling on developers than there is on designers. So maybe the average developer makes more than the average designer (again a hunch), but I'd bet there are a select few really great designers making MUCH MUCH more, and I don't think the same is true for developers.

I mean, what's a really stupendous developer going to do for you that makes it worth paying him the equivalent of a 5-man team? a 10-man team?

We could make an analogy to construction. The best construction company is no doubt going to build your building faster, more efficiently, with fewer problems, but that's not going to triple the value of the finished product. The architect by contrast can make the difference between a world-famous landmark and just another good-enough building.