Because SDO takes a lot of pictures of the Sun. At perihelion they appear a little bigger than they do at aphelion in July. Here is a comparison of HMI Quick-Look continuum images from January and July. The two horizontal lines show how high the Sun appears today. When those lines are extended to the left, the July image is a little over 3% smaller. When we designed the instruments on SDO we had to make sure the largest appearance of the Sun would fit on the CCDs.
Looks like we did a good job!
Have a Happy Perihelion Day!
