Limits in AcuXbis for Extend

We have been using xBIS on Windows for Extend 9 for some time.  We're implementing a new method which requiring uploading quite a bit of array data.  A curious thing happens - if the content length is 49152 bytes or less, the BIS Content section is populated and everything works fine.
HTTP_CONTENT_LENGTH:49153
HTTP_CONTENT_TYPE:text/xml;charset=UTF-8
HTTP_ACCEPT_ENCODING:gzip,deflate
HTTP_HOST:optics2012
HTTP_USER_AGENT:Apache-HttpClient/4.1.1 (java 1.5)

With the exact same schema, but different data such that the content length is 49153 or greater, the BIS Content section is EMPTY, and obviously the method fails.

It seems there is a limit of 49152 imposed somewhere, but we can't find an entry in IIS anywhere.

Does this sound familiar to anyone and if there is a limit, is it imposed by BIS or can it be relaxed?

Thanks

Tony