Actually, settlment-free peering is pretty much exclusively about the amount of transit happening in either direction. Most peering/depeering disputes between Tier-1 providers (and sometimes their customers) tend to happen over one side thinking that they are receiving a great deal more transit than they are passing on to the other end - that the other end is "getting a free lunch", so to speak.
All peering is, by definition, settlement-free. If the difference is measured in orders of magnitude, it's certainly not unmeasurable, in which case depeering is inevitable. In a large network, it's very difficult to determine
precisely how much bandwidth is being used. One can easily measure how much data customer X is using, but accurately monitoring how much traffic is going across the connection that is represented by the peering agreement is a feat that large ISPs would pay a lot of money for.
I imagine that an accurate solution would result in the abolishment of most peering agreements, in exchange for a pay-as-you-send agreement for the same link, thus eliminating the example you gave.
