There seems to be a false alarm spreading over the online forums that WSO2 ESB cannot handle messages larger than 16K and results a corrupted response. This situation occurs only when a user enables one of the synapse properties which is disabled by default.
Streaming XPATH option is the aforesaid culprit, which is a newly introduced feature to optimize the performance in different message filtering / routing scenarios. However this does not effect the default workings of WSO2 ESB which does handle large messages successfully in thousands of enterprise deployments with the default XPATH implementation that ships out of the box.
As any new component achieve perfection with time, WSO2 ESB team have fixed all reported issues against streaming XPATH component (in ESB v4.8.0) and thrives to be the fastest ESB and the integration platform.
Following case study provide mode details on large message handling of WSO2 ESB in production.
UPDATE (02/17/2014) : WSO2 has released the latest performance stats, which details on the stream XPATH stabilization as well.