Zvezda currently holds this honor, at 20.3 tons. Nauka’s future docking port at the ISS – Zvezda nadir, or Earth-facing, docking port – is currently occupied up by the Pirs Docking Compartment.

Galley, Space Vehicle Mockup Facility, NASA Johnson Space Center, Texas Equipment lock and crew lock with extravehicular activity hardware, ISS, Low Earth Orbit View of ISS Nadir from Departing Soyuz ...