Serving static with nginx and varnish

I used nginx as reverse-proxy in front of amazon s3.

A month ago I decided to try varnish. It is designed from the ground up as a reverse-proxy. Also, I though that nginx solution wasted a lot of resources when keeping lots of tiny images in separate files.

But, after month of experiments, I discovered high iowait values and severe load on the hard disk, causing service problems. I rolled back to the previous nginx static scheme. Iowait dropped from frightening 100-150 to acceptable 25.

I used varnish 2.0.4 running with 3GB file storage. It consumed 0.5-1 GB of memory. Does anyone have a clue why varnish performed so much worser than nginx?