I was originally going to leave this as a comment at Untimately: Hexagram Advancement Draft, but it got kind of unwieldy for a comment.
As Brendan says, most people increase challenges and rewards to keep advancement at about the same real-world rate at every level, but let's imagine for a second a campaign where those bigger challenges and rewards just don't exist... What you'll see is fast advancement at low level, and slow advancement at high level that eventually tapers off to nothing. Every adventure will pull in, say, 8000xp, but if you need 1,000,000xp to get to the next level, that's basically never going to happen.
In something like the real world, you can only get so good at fighting - say about 5th level in D&D. If you want to get more powerful than that in real life, you have to become a warlord, baron, king, etc. - which is pretty much what was sketched out in OD&D, B/X, and BECMI. I think the fact that there's levels in the game beyond a certain point (I don't know what that is, but between 5 and 9, probably) is the real illusion - you shouldn't practically be able to get to those levels. And, if you play in a campaign where there aren't dragons around every corner, you won't.
Now, those systems allowed you to use domain-level play to increase your fighting capability (IIRC), whereas I think only gold found during adventuring should count for XP, but that's another issue. The point is, the old systems could be looked at as attempting to place the higher levels practically out of reach, and thus enforcing a logarithmic progression.
I think this might be one of those things that was less broken (or just differently broken) than people thought it was, and that they house-ruled or campaign-designed into a problem.
In fact, I think the notion that all of the levels of the game should be accessible might be the dawn of the level-appropriate encounter dogma...