Why does the sun appear 13x bigger through diffraction in Voyager 1 image from 1990, but not from Earth? The sun in the Voyager 1 photo here from 1990 appears 13x bigger than its diameter, diffraction of course. On Earth, photographs of the sun do not make it appear 13x bigger. Why?

A description of the photo from jpl.nasa.gov,


  "The wide-angle was taken with the camera's darkest filter (a methane
  absorption band), and the shortest possible exposure (5 thousandths of
  a second) to avoid saturating the camera's vidicon tube with scattered
  sunlight. "
Don't base the size of the Sun on what you see in that image because the Sun is over-exposed.  The big spot in the center of the diffraction spikes are saturated pixels because they all appear to have the same value (the spot looks uniform) so you don't know what their real values are; you would need to take a shorter-exposure image (or put filters in front of the lens) if you wanted to take a proper picture of the Sun.  Since Venus is a lot dimmer than the Sun, they set the exposure long enough to see it and accepted the fact that the Sun would be over-exposed.  You see the same thing in some of the Hubble images where to bring out the background stars, the brighter ones are over-exposed.

Another way of looking at it, if you see an image where you can very clearly see the diffraction spikes, that part of the image is over-exposed.