Thats because the microgravity environment allows room for their vertebrae to expand.
Heres how NASA explains it:
Imagine that the vertebrae in your back form a giant spring.
Pushing down on the spring keeps it coiled tightly.
When the force is released, the spring stretches out.
In the same way, the spine elongates by up to 3% while humans travel in space.
However, Kelly should return to his regular height after spending some time on the ground.
Kellys height change was especially striking when he stood to his identical brother, former astronaut Mark Kelly.
Its not statistically valid to say that differences between them must be due to the spaceflight.
However, being in space comes with a host of negative side effects.
Astronauts and cosmonauts can suffer a loss of vision.
Their muscles can decrease, bones shrink, and their heart too may weaken.
They may also be damaged by radiation.
Theyre turning humankind from an Earth-dwelling species into a space-exploring species.
One day, humankind will be a species that can settle on other planets.
Its a multi-generational effort.
source: www.techworm.net
