Jim Gettys wrote an blog entry where he describe problems with a oversized router buffer - he called the problem bufferbloat. Buffer in network equipment exist to catch temporary load peaks and allow proper internal packet exchange from one linecard to another linecard in coexist with the max. bandwidth.
In the case where buffers are overestimated the effect get worse during periods of network congestion. Overestimated buffer will shift the problem in time, or in other words: TCP will overestimate the available capacity and normal congestion avoidance mechanisms do not timely take effect. Overestimated buffers concrete shift the congestion avoidance mechanisms in time - packets are dropped to late and the packet drop is not aligned with the link capacity but rather with the buffer space. The problem with the artificial delayed congestion avoidance is that the congestion avoidance mechanism will overestimate the available bandwidth therefore. User impact is a high latency and jitter. For a proper functioning the mechanism must take promptly and should reflect the link capacity.