On Earth, the spinal disks are slightly compressed due to gravity. In space, that compression is no longer present causing the disks to expand. The result: the spine lengthens, and the astronaut is taller. Read more about Scott Kelly’s year in space.
NASA’s Scott Kelly Grew 2 Inches: The Body After a Year in Space
By: Dr. Felix Gussone and Dr. Shelly Choo
After living for nearly a year aboard the International Space Station, NASA astronaut Scott Kelly is two inches taller than his identical twin brother Mark.
One of the main goals of his groundbreaking mission is to study how well humans can endure — mind, body and spirit — on a long-duration spaceflight.
Source: http://www.cnbc.com/2016/03/02/nasas-scott-kelly-grew-2-inches-the-body-after-a-year-in-space.html