The key variable is "recording density":
In order to maintain the same or similar recording density,
the armature's write heads must write binary digits ("bits")
at a frequency that is faster at outer tracks and
slower at inner tracks.
Thus, there is a step function in the recording logic,
which gradually reduces the bit recording frequency
as the armature moves from outer to inner tracks.
You can illustrate this yourself by drawing a series
of concentric circles, and placing equally spaced dots
along those circles: the inner tracks will have fewer
dots if all dots are equally spaced, relative to
adjacent dots.
You can also see a graphic illustration with the HDTune
benchmark software:
http://www.hdtune.com
Western Digital have an excellent animation here:
search for "Intelliseek":
http://www.wdc.com/en/products/products.asp?driveid=338
then "View demo":
http://www.wdc.com/en/flash/index.asp?family=intelliseek
The inner tracks obviously have a shorter circumference:
in geometry, circumference of a circle = Pi x diameter
or 2 x PI x radius, where "Pi" = ~3.14159265
MRFS