I don't know how you do it but when sending data from a file, I read chunks of the file and send the chunk I have read, not the entire thing. These chunks can be very small, particulary as the MTU over a network is something like 1500 bytes (unless using jumbo frames). So to sync 10 files in tandem (10 threads), they would need 10 * 1500 bytes for the read buffers (+ overhead for storing the file pointers). Even you can see that it is a tiny amount of RAM required for this.
Or are you somehow living in a world where your internet upload speed is somehow faster than reading from disk or memory, and your network interface has to wait for reading from disk/memory???
Are they reading the synced files entirely into memory or something stupid like that?
If you want Dropbox to reduce memory use will you start complaining that they’re fixing fewer bugs or syncing is taking longer?