Salmon tends to shrink up immediately when it hits a heated pan. You will actually be able to tell that your pan is heated properly if the salmon shrinks as soon as you put in on. Lots of fatty types of fish like salmon tend to do this when they hit the pan.
Despite what many people think, salmon shrinking when it is put on a heated pan is completely normal and a sign that your pan is heated to the proper temperature and you are ready to start cooking it. Just make sure that you cook your salmon through all the way so as to avoid any potentially harmful parasites or bacteria that could get you sick if you eat it fully or partially raw.