why does the sun look bigger and paler during sunrise and sunset when its relative distance remains constant?

Answer 1

5Cents has it mostly correct.

The curvature of the atmosphere makes it so that it acts like a lense, distorting the size of the sun (and the moon, and the stars, and anything else) the closer the sun gets to the horizon, because the light from the sun is being refracted through a much thicker cross-section of atmosphere than when the sun is directly overhead.

It looks paler because of the greater quanity of dust particles and vapor particles in the atmosphere blocking the sun's rays.

Answer 2

Ignore these other answers.
Your mind is playing tricks on you!
The Sun looks bigger because of an optical illusion. When the Sun is near the horizon, it sometimes seems to be much bigger and closer than during the rest of the day. At Sunrise or Sunset, your brain has other objects to compare the Sun's size to, such as trees and buildings.

Answer 3

Because the sun is being viewed through the prism of the atmosphere. Because the sun is at such a shallow angle relative to the Earth at sunrise and sunset, the light has more atmosphere to pass through before it gets to our eyes. The light is diffused and scattered, thus making the sun appear bigger.

Answer 4

The view in which it is seen from the atmosphere in the morning and evening sort of "magnifies" the shape and size relativity. Therefore it is seen differently when on the horizon than directly above.