Excellence, thanks for the illustration. That is exactly what I thought: you're looking at logarithmic scales. It would help if they put the numbers along the bottom of the x axis. If they were to do that, you would see that what you call "compress and rarify" really isn't that at all. Things keep on compressing the same throughout---but periodically they just draw the grid lines less frequently so they don't get too crowded.
So for example, when you see a frequency response chart for, say, 10Hz-10kHz, the grid lines are drawn at the following places:
10, 20, 30, 40, 50, 60, 70, 80, 90,
100, 200, 300, 400, 500, 600, 700, 800, 900,
1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000,
1000.
So what you see as "compress and rarify" is just the fact that they jump from drawing the lines every 10Hz to drawing them every 100Hz, and then again every 1000Hz.
The distance between two points on a logarithmic frequency axis is proportional to the ratio between the frequencies. For example, if you took a ruler and measured the distance on this graph between, say, 30Hz and 3000Hz, this distance would be the same as if you measured the distance between 50Hz and 5000Hz, because both have a ratio of 100.
Since our brain interprets frequencies in this fashion, this is a sensible way to draw frequency response charts. No matter where they are along the x-axis, features on the chart (dips, peaks, etc.) are represented in a proportion that corresponds to the way we hear them.