Turns out it was a problem with the PureFTP config. The server server was configured to return up to 50000 items for an ls command, but was only returning 2000. changing the value to 10000 in the .conf file fixed the issue, at least until they have more than 10000 images...

I have a situation where synchronizing a local folder to the server (direction remote) always copies all the files. My past experience and understanding from the documentation tells me that synchronize should get a list from the remote server, compare the file sizes (or time stamps) and only copy what's changed to the server. Is this correct? The primary purpose of this post is to clarify my understanding of how this is supposed to work.

I'm using file size for compare, and in the WinSCP client the local and remote file sizes are identical (as displayed by the client) for the files synchronize is choosing to copy to the server. The remote server is PureFTP, the version of WinSCP is 4.2.8. I am doing the synch through the UI, not a script at this point.

This is a big deal, because the local folder has approximately 7,000 jpegs in it, and as you might imagine, copying 7000 files to the remote server takes more than a few minutes over a fractional T1 connection...