Ever seen a picture of a blobfish in it's deep ocean habitat vs when it is at the surface? It's body is adapted to the extreme pressure of the deep sea, and when that pressure is no longer there, the forces keeping it's shape are no longer present and thus every bit of it expands. That's what would happen to us in the vacuum of space albeit on a lesser scale. Also, we're like 70% water which boils in a vacuum.