I have observed the asymmetry you described.

More formally, let 's' equal the send capacity, and 'r' equal the receive capacity.

A true asymmetric connection's relationship between send capacity and receive capacity can be defined using the following function:

r = f(s)

f(x)=C

where C is a constant.

Some providers, in an attempt to lower bandwidth costs, frustrate p2p applications by changing f(x). In the simplest form, f(x) can be redefined as:

f(x)=C1 for x<=X0
f(x)=C2 for x>X0

where X0 is the send cap and C2 is much less than C1.

This allows normal users who are surfing or downloading to get the maximum throughput, but penalizes those who use a lot of upload bandwidth (as those are running p2p apps or servers).

Usually, f(x) is a smooth function which is close to C1 for most of the range of x, and drops off sharply at the upper limits of x (the send limit).