"We had our bodies measured after reaching space, and wow, wow, wow, I had actually grown by as much as 9cm!," he tweeted. "I have grown like a plant in just three weeks" and "I'm a bit worried whether I'll fit in the Soyuz seat to go back."
That launched tens of thousands of reactions on social media, with many Japanese wanting to go into space to be taller.
Kanai stood 1.8 meters when he left Earth compared to the 1.71m of the average Japanese male.
It's a fact astronauts' spines can extend in the zero gravity of space, making them taller. But they return to normal when they go back to Earth.
Kanai had in fact worked himself into a tall story. He measured himself once more after a Russian astronaut told him a 9cm spurt was unlikely and found he had grown 2cm.
He has now tweeted apologies for what he said was "fake news," adding: "I'm a bit relieved as I'll probably fit into the Soyuz."