The so called "optimal size" is a compromise that gives the "best" resolution with the least diffraction. If you made it smaller, you'd have sharper resolution, but more diffraction effects. To make it larger, you'd get more mush from the decreased resolution, but the diffraction would not affect the image as much. Of course, what "best" and "worst" mean are dependent upon what you want. I happen to prefer greater resolution with greater diffraction to a certain extent. This is because I enjoy shooting into the sun. Another aspect of the "optimal" is that it isn't the same for all wavelengths of light. Since different colors diffract more or less. Red diffracts less (that is, changes direction less) and blue more. This is why a diffraction grating works. Generally, a region in the green portion of the spectrum is chosen for these calculations.

Almost universally, these two factors are rarely considered each upon its own, but are assumed to be simply defects having equally degenerative consequences for the image, but the images you can get by adjusting the resolution/diffraction balance will not be the same. You get to choose your defect.