Windows TCP buffering

After changing the send buffer size for FTP upload, I thought I would check if SFTP upload mentioned in part 1 also suffers from the same limitation. To my surprise, SFTP uploads were able to push data continuously, even though the send buffer size was not set anywhere in my code.
An experiment showed that the SendBufferSize property after SFTP upload usually contained a different value from the original 8KB; for FTP upload, the value always remained at 8KB. The only apparent difference in code was the application buffer size - SFTP application was sending ~32KB batches, while FTP application was sending 8KB batches all of the time.
I increased the batch size on FTP application side. Immediately it became apparent that the entire batch was transmitted over the wire before waiting for an ACK, which improved the send speed. Furthermore, with batch size of 32KB the SendBufferSize was also automagically raised behind the scenes, further improving the send speed.
I couldn't find any documentation for the observed SendBufferSize changes. The effect of application send buffer on transmission speed was described in this SO question and this MS Knowledge Base article. To summarize, if the application send buffer is the same or larger than the socket send buffer, only 2 pending send operations are allowed at a time. However, the socket can buffer whatever the application passes, even when the application buffer is larger than SendBufferSize, so using a larger application buffer can speed up the transfers the same way using a larger socket buffer can.