Isotonic saline and 5% dextrose in water are solutions that are considered Isotonic to human blood. ...

a medium in which cells are placed is said to be isotonic to the cytosol if it contains the same concentration of solutes. If the surrounding medium is diluted with water its solutes concentration will decrease and thus becomes hypotonic. When solutes concentration are higher in the surrounding medium, the medium would be called hypertonic.

So you see when dextrose concentration increases the medium becomes hypertonic ( containing more solutes, which is dextrose in this case ).
If the solution is isotonic to human blood then nothing will happen since an equilibrim in solutes concentration is already achieved.
But when solutes concentration becomes higher in the medium around the cell, the cell will seek to reestablish tonicity by allowing water from inside the cells to diffuse towards the outside in order to dilute the solutes present outside to restore the equilibrium.. the diffusion of water out of the cells will cause their shrinkage.