Start with the picture
Draw one bar and shade half of it. That shaded half is the amount being shared. To take half of it, cut the shaded region into two equal pieces — each piece is now one of four equal pieces of the whole bar, which is 1/4.
The pieces shrink because sharing divides the size of each piece, not the number of wholes. One half shared between two people gives each person a quarter: smaller than the start, exactly as sharing should be.