Osmotic pressure and turgor pressure details
when we talk about external force applied in this case we talk about a setup that is made by us. like a U tube experiment that is filled with water and separates both the hands of the tube via semipermiable membrane and we add solute one side and due to this water starts to rise in that hand of the tube to avoid this rise in water level an external pressure is applied tht prevents this water rise and this pressure is termed as osmotic pressure. THIS IS THE CASE OF A ENVIROMENT WHERE EXTERNALFORCES CAN BE APPLIED BY US AND IT IS VERY MUCH DIFFERENT FROM THAT OF THE CELL WORLD.
in case of cells osmotic pressure is directly proportional to the concentration of the solution i.e more the solute more the OP and visa versa. in cells osmotic pressure are internal force that is caused due to  difference in concentration of the solutions.
and as we know water move from low osmotic pressure to high osmotic pressure
{ from less concentrated to more concentrated solution} thus cell swells up because of entering of water when cell is placed in pure water or hypotonic solution.
CONCLUSION : CELL DO NOT SHRINK BECAUSE NO EXTERNAL FORCE IS APPLIED BY US THE DEFINATION U GAVE IS CORRECT BUT IT IS NOT APPLIED IN CELL WORLD. IN CELL WORLD OP IS A PRESSURE DEVELOPED DUE TO DIFFERENCE IN CONCENTRATION OF SOLUTIONS AND THIS PRESSURE IS CREATED INORDER TO EQUAL THE CONCENTRATION OF SOLVENT (WATER) IN BOTH THE SOLUTIONS.