You get due to you have many Source documents and the total message returned is larger than 65536.
This limitation is the default behaviour by Microsoft. However you can modify is by changing the MaxRecievedMessageSize on the binding to API (see https://msdn.microsoft.com/en-us/library/system.servicemodel.basichttpbinding.maxreceivedmessagesize(v=vs.100).aspx)
Also in all QlikView services you can change this value in the config file per executable like this (QlikView a higher default value of 262144):
<add key="MaxReceivedMessageSize" value="262144"/>