dd surprise

I was testing the speed of external USB harddrives and ran across something that surprised me.  I have assumed for a while that increasing the block size while doing a dd improved throughput but I tested anyway, and it didn't.  Here are the figures for not specifying block size, and then using 500k, 1m, and 500m:

 10,460,172 bytes/sec
 10,385,009 bytes/sec bs=500k
 10,529,657 bytes/sec bs=1m
  9,573,095 bytes/sec bs=500m

Looking at the overall speed of USB 2 (it's 480Mbps or 62,914,560 bytes/sec), it appears that it's USB that is the limit in this case.  We're getting not quite 20% of theoretical.  This on a MacBook Pro running Snow Leopard.