No, this is not about mutual traffic -- this is about benefit to
_a_ customer.
Your computation is correct but is completely irrelevant from the
point of view of a customer. They're interested in reacheability, not
traffic at some exchange point.
I.e. each small ISP's customer derives 99.9% of benefit from the peering,
while large ISP's customer derives only 0.01%. Hence, the large ISP
can force small ISP to pay up -- without connectivity to a large ISP the
small ISP will be dead pretty soon. This is exactly like grocery chains
which can (and do) force consumers to pay up for food -- although you
can grow your own tomatoes in your backyard and probably sell them to
those grocery chains. Good chances are, they're not interested.
--vadim