It's well known that astronauts grow a bit taller in the weightlessness of space, a result of the spine decompressing.
Mr Kanai tweeted: "Good morning, everybody". "Today there is a serious report". I've had physical measurements since I got to space, and, wow, I've grown by up to nine centimetres. "In only three weeks I've really shot up, something I haven't seen since high school", he originally tweeted.
"It appears I can fit on the Soyuz, so I'm relieved".
Along with astronauts Scott Tingle and Anton Shkaplerov, Kanai arrived at the ISS on 17 December 2017 after launching from the Baikonur Cosmodrome in Kazakhstan.
He quickly re-measured himself after a Russian colleague questioned the measurement and found that the growth spurt was actually less than an inch - well within the range of what astronauts usually experience in space.
The absence of gravity allows the vertebrae in the spine to spread apart in order to grow in height or stretch out.
"Nine centimetres is a lot, but it is possible, knowing that every human body is different". He is due to return to Earth in April, giving him three months for another spurt.
Clayton Anderson, a former NASA astronaut, told ABC News that getting taller in space is normal.
After an influx of media interest-and questions from his captain-Kanai made a decision to remeasure himself to find out how tall he had actually become. While growing slightly taller is common in space travel, Kanai reported a massive increase of 3.5 inches in only three weeks.