Astronauts actually grow while they are out in space. This growth is due to the lack of gravity when they're in space compared to when they are on Earth. Earth's gravity naturally compresses the spine when people stand, making them slightly shorter over time. In space, the spine lengthens and decompresses as the astronaut spends time in space. This often results in the astronaut coming back to Earth taller than they were when they left, sometimes even by up to a few inches.