Now that you mention config file, the only thing I did change there, and was suggested to me while I made some on my databases was "max_locks_per_transaction = 50000" (which has default value 10000).
After resetting "max_locks_per_transaction" to default value and restarting the server, memory occupied in working set reduced linearly to around 200 MB.
I'm not an expert on the Windows version, so I could be off-base, but the POSIX versions of Postgres allocate shared_buffers worth of memory at startup and lock it for exclusive use by Postgres. Do you have shared_buffers set to around 1G, perhaps?