Interesting question. Isn't mortal magic (at least) partially powered by human emotions? Harry frequently discusses that certain areas have residual energy left over from emotional events, though this could be his Third Eye interpreting the past in his peculiar way—I recall that most, if not all, of the times Harry's mentioned this effect were ones where he was using it. Though black magic is outright stated to be affected by negative emotions—it's the reason the Kemmler disciples or Cowl blacked out Chicago in Dead Beat.
If it's true that human emotions provide energy, then the simple increase in numbers indicates that there is more energy. Whether or not this energy is actually useful is another question. I think of it as ambient radio waves; there are plenty that are naturally occurring, but since we started sticking up towers, there are a lot more that do specific things if you tap into them.
In terms of power dilution, I think that's more a side effect of changing beliefs. Hades still has power where he is because they still tell stories about him; ditto Odin. They're not what they once were, but they're still leagues ahead of Harry. Mab apparently sponsored the Brothers Grimm to keep her in the public eye, and Harry speculates that she hooked up Walt Disney later on. But, if power can't be created or destroyed, only changed, that might explain why Odin has an extra mask (in his terms, from the end of Cold Days) in Kringle; he's transferred some of his power into something new.
Here's a tangential, but related question: Bob is a spirit of intellect, right? He grows in power when he knows more. Did he get a power boost when Butters hooked him up to the internet and connected him to what amounts to the collected sum of human knowledge? Is that why he was able to do all of those other things Harry never considered when Butters used him to fight Binder's mooks? Or is that just an expression of Butters thinking outside the box?
