"I have a major announcement today", Kanai wrote in Japanese, as translated by the BBC.
Norishige Kanai told his social media followers on Twitter that he is now anxious he might not fit into the seat of the Russian Soyuz vehicle that is due to bring him home in June.
Norishige Kanai, waves during a send-off ceremony at the Russian-leased Baikonur Cosmodrome in Kazakhstan early on December 17, 2017. I've had physical measurements since I got to space, and, wow, I've grown by up to nine centimetres.
This is a known phenomenon that astronauts "grow" during the space missions because their spines extend in the absence of gravity, but the gains are generally limited to a couple of centimetres and disappear once they are back on the ground.
He joined five other worldwide astronauts in December for Expedition 54 to the global Space Station and is orbiting Earth 250 miles above. "This is the most I've grown in three weeks since junior high school". "As a matter of fact, the tightness I had in my shoulders is gone so I'm pretty certain I did not grow 9 cm". I'm a bit anxious whether I'll fit in the Soyuz seat when I go back'.
And when astronauts return to earth, so too do they return to their usual height.
Libby Jackson of the UK Space Agency explained his growth was more than expected but did not express concern he could not return to earth.
But first, Kanai, who is spending his time as a flight engineer aboard the ISS - an Earth-orbiting satellite - as part of the Expedition 54/55 crew is "helping doctors understand how being away from the normal 24-hour sunrise-sunset cycle impacts the human body", says NASA.
The 41-year-old Kanai, a former Maritime Self-Defense Force doctor and the 12th Japanese to travel into space, said he went back to the tape measure after fellow astronaut Anton Shkaplerov of Russian Federation questioned his newfound height.