I don't think this is a bug in the WinSCP program, because I get the same problem when using a different SFTP program.
Any time I download using SFTP, my physical memory gets used up as I download, and eventually windows runs out of memory and crashes.
I've tried disabling prefetch and superfetch, and I also have disk caching turned off on all of my HDD/SSDs.
I'm using windows 8.1, which is totally up to date, as are all of the SFTP programs that I've tried downloading with. (latest version of WinSCP GUI in Commander mode)