Why does fresh spinach get smaller when you cook it?

When I cook fresh spinach (boil, steam or saute) I can fill a pot/pan to the brim but as soon as it heats up the spinach loses 3/4 of its volume as it wilts. What makes this happen? Is it a chemical reaction within the leaves? Are the leaves themselves getting smaller by removing air or water from them? Or are the leaves keeping their m and just getting less rigid, eliminating the air spaces between them? Maybe a little bit of both?