Ok I found it.
The Woodstox parser, used by CXF, does not return more than 500 bytes when the element is of type javax.xml.stream.XMLStreamConstants.CHARACTERS.
(in fact, a 'TOKEN_PARTIAL_SINGLE' value is returned).
com.ctc.wstx.util.TextBuffer
final static int DEF_INITIAL_BUFFER_SIZE = 500;
Now that I know this, I will make sure no such kind oy payload will be sent...
Regards