Input and Output data traffic

Hi,

I'm doing some benchmarks for a paper in my university, and with the help of
a proxy I observed something strange.

The volume of sent data (application -> database) and the response data
(database -> application) are basically the same.

For simple inserts/updates/deletes i got a difference of about 15% between
sent and received data.

I used a ruby driver and got the same results for the same instruction.

Finally I used a Java JDBC driver (for postgres too), and the difference was
huge, it was 70% (the received data was much smaller).

Do you guys have any idea about what could be? Is it possible to the JDBC
driver is using some kind of data compression for the traffic?

Thanks in advance,

Israel