It seems like this info should be front in center in the test. 1M/s 100kb writes is much more impressive than 1M/s 16 byte writes.

That said, there's a previous benchmark linked to at the top of the post:

http://techblog.netflix.com/2011/11/benchmarking-cassandra-s...

The client is writing 10 columns per row key, row key randomly chosen from 27 million ids, each column has a key and 10 bytes of data. The total on disk size for each write including all overhead is about 400 bytes.

There are 3 replicas, so figure that in as well.