Cyberduck 5.0.1(also 5.0.2, 4.9.x, and 5.1(beta)) is using 800MB~1.4GB of memory even on launch. I don't think ftp program needs more memory than photoshop. I tried to uninstall completely and install again, but the result was the same. There was a similar ticket(#8912) and the resolution comment stated "it requires 200MB after launch".
I also reported more than once on this. Same problem which makes Cyberduck troublesome for me. MACOS 10.11.6 and Cyberduck 5.1.0.
I have 57 bookmarks, but even without any opened Cyberduck uses 1.4GB of memory.