Yeah I get the count vs area distinction, but I think the point would have been more cleanly communicated with e.g. a stacked barchart with failures by quarter. With circles, the filled area over any span of the x-axis is not solely due to failures which happened during the corresponding time period. So the area above a short span in early 2010 includes area contributed by WaMu, which happened in Sept 2008.
It is the total volume of money involved vs the count. The height of both stacks show just how much damage three did vs all the others in the left hand stack.