Most astronauts grow taller during long space missions – albeit not usually by as much as 9cm.
This is down to the fact that, without gravity pressing down on them, the vertebrae in their spine begin to separate and elongate.
Mr Kanai, an astronaut and researcher on board the International Space Station (ISS), had expressed concerns about whether he would be able to fit in his seat to return to Earth.
He tweeted: “My height’s been measured here in space and somehow, somehow, I’ve grown 9cm! In only three weeks I’ve really shot up, something I haven’t seen since high school.
“This makes me a little worried that I might not be able to fit in the Soyuz seats for our return.”
However he was forced to apologise as it emerged he had not stretched by that amount at all.
After his captain raised questions about how much he had grown, Mr Kanai re-measured himself and found he had only grown by 2cm.
He tweeted again: “This mis-measurement appears to have become a big deal, so I must apologise for this terrible fake news.
“It appears I can fit on the Soyuz, so I’m relieved.”
It is not yet clear how the original miscalculation occurred.
Mr Kanai travelled to the ISS three weeks ago as part of a six-month research trip.
During their time in space, astronauts would normally expect to grow by 2-5cm. For example when NASA astronaut Scott Kelly returned from the ISS after a year-long stint, he had grown by 5cm.
When astronauts return to Earth, however, they return to their normal height.
When the news first broke, Libby Jackson of the UK Space Agency told BBC News: “Nine centimetres is a lot, but it is possible, knowing that every human body is different.
“You do get taller in space as your spine drifts apart, usually by about two to five centimetres.
“There’s a range of growth for different people, and everybody responds differently.”