The difference (systemic vs non-systemic) is not just about magnitude, but also in who Lehman's counterparties were and the effects when those counterparties (those who were exposed to Lehman) could not get their expected capital (and the chain reaction of events that sets off throughout the capital markets and then the real economy). I remember reading some stat that claimed Lehman had derivative contracts with total notional value in the 10s of trillions.