This is analogous to mixing 2 full cups of rocks with 5 full cups of sand.
Although 2 cups + 5 cups = 7 cups, what you get will all fit in only 6 cups.
That's because rocks are larger than grains of sand, and the grains of sand
fill in the spaces between the rocks.  It's the same principle with mixing,
say, water and ethanol. Water molecules are larger than ethanol molecules.  

Edwin