Tomcat 7 Valve not logging correct response size

[Antonios Kogias]

Good morning,

  I'm using Tomcat 7.0.16 and a Valve in the server.xml file that uses 
the %B option to log the "Bytes sent, excluding HTTP headers" 
(http://tomcat.apache.org/tomcat-7.0-doc/config/valve.html).

<Valve className="org.apache.catalina.valves.AccessLogValve" 
directory="logs"
               prefix="localhost_access_log." suffix=".txt"
               pattern="%H %p %m %D %s %B %a %t &quot;%r&quot;" 
resolveHosts="false"/>

This works correctly for small response size (up to 30-40 kB), but for 
bigger sizes it doesn't; it only writes zero as response size in the web 
access log (I have tested that with 100 kB and 1000 kB static files).

Any idea why is that happening and what can be done to overcome?

Antonios

PS. OS is MS Windows XP 32bit SP3