What is the advantage of red blood cells being approximately the same diameter as blood capillaries?

The function of red blood cells is to transport oxygen around the body. Being of a similar diameter to blood capillaries, red blood cells must bend and squeeze through the capillaries one at a time. Firstly, this slows the flow of blood to increase the time available for gaseous exchange and diffusion of oxygen out of the red blood cells into the surrounding tissues. Secondly, it reduces the distance that oxygen needs to travel as the red blood cells are pressed against the wall of the capillary and thus increases efficiency of diffusion. Thirdly, the bending and stretching of the red blood cells through the capillaries increases the surface area available and improves diffusion of oxygen into the bodily tissues.