The volumes of potato pieces and of an egg without a shell (left photo) increased  very much when they were placed into water.
This observation can be explained if you remember the experiment showing that  water molecules are in quick and continuous movement.
* In the right photo you can see triangles and small dots surrounded by the egg membrane.
* The dots are models of the small water molecules,  the triangles are models of the large protein molecules dissolved in water.
* Outside the egg membrane only water molecules are present. The membrane can be compared with a screen:
  Due to their small size and great speed water molecules can easily pass the  meshes of the screen.
  Bigger ions and molecules of proteins are prevented from passing these meshes.
* Because more water molecules are outside the membrane than inside more and more of them enter the egg resulting in an osmotic pressure.
* Explanation:                Look under osmosis.