Rise until doubled means until it has doubled in size. As it is a yeast mixture the yeast gives off a gas which is trapped in the dough and keeps expanding. Unless you are very unlucky [and if you follow the instructions all should be well] these air pockets are evenly distributed throughout the whole dough which causes it to rise until it is about double the size it was when you set it to prove. It won't be hollow inside, just hundreds of tiny cells of air which, when baked, cause it to be light and fluffy.