Well, the bones in our spines legs and feet get smaller because we don’t use them in microgravity as much as on Earth. It can be as much as 1-3% for each month in space- which is rather a lot. Strangly the bones in the arms and head sometimes ( but not always) get slightly bigger. We think this might be because astronauts have to use their arms more in space- after all they are being used as arms and legs. No one is quite sure what’s going on with the head though!
The astronauts exercise everyday to try and keep their bones as strong as possible.