Adjusting the IIS setting for uploadReadAheadSize to 5MB (5242880) resolved this issue and allowed the ADScan audit file from the Gateway as well as the larger Software Inventory Audit files on client agents to be sent in and processed.
For this particular customer 5MB was plenty of space for the largest messages we were seeing. This can be set higher or lower as long as this setting (in bytes) is larger than the biggest audit file attempting to be sent in. (You can easily see this in the Fiddler Trace above as the "Entity / content-length" in the Request Headers section.)
To check value of uploadReadAheadSize
• Open IIS
• Navigate under Default Web Site (Wherever ClientTransportProcessor application is present)
• Scroll down to Management and open Configuration Editor
• Select following section (drop down at the top) system.webServer and expand it, then locate serverRuntime
• Check current value of uploadReadAheadSize value.