Why the sun looks bigger during sunrise and sunset?

i guess it's just an optical illusion (blending of colors, gradient effect)

This observation applies equally to the moon. The moon also looks much bigger when close to the horizon than when it is high in the sky. The consensus is that this universal experience has nothing to do with the physics of seeing or the size or shape of the image. You can easily check that the actual size of the image is the same, independent of the location in the sky. This is very conveniently done for the full moon using nothing more than a transparent scale held at arm’s length with one eye closed. No doubt, you will convince yourself that the actual size of the moon does not change as the moon climbs up in the sky (A NOTE OF CAUTION: IT IS HAZARDOUS FOR THE EYES TO TRY THIS WITH THE SUN, UNLESS YOU USE A CERTIFIED FILTER!!!). In any case, you can take it from me that physics is not fooled by the proximity of the sun (or the moon) to the horizon. But we are. This optical illusion is fundamentally linked with the way our brain interprets images. It uses past experience - some facts, and some prejudices — to give meaning to the visual signals received in the cortex. We are aware of a large number of optical illusions. But the field of psychology is not as certain, or crisp, as mathematics. Two possible explanations have been put forth for this observation; see if they satisfy you. When the sun, or the moon, is near the horizon we see it in the company of other distant objects — buildings, trees, and hills — objects whose sizes our brain is familiar with. These distant objects produce a small image, but the brain, from past experience, applies a process of mental amplification, which justifies and rationalises the sizes of these known objects. The error occurs when the brain applies a similar amplification strategy to size the “adjacent” sun or moon!

I think it has something to do with that it's closer to the horizon when it rises and sets so it looks bigger. It seems like it's closer to the earth, but it's really not.

When we stand out in the open, very far from anything else, we get an impression that the sky is an inverted bowl. This has been felt and believed for thousands of years. We do not know the height of this bowl, but get a feeling that it cannot be as much as the distance to the horizon. After all, we know that the horizon, where the rim of the bowl seems to meet the ground, must be very far because known objects look so small when they move close to it. So it seems natural for the intelligent interpreter sitting in our brains to make the sun and the moon appearing near the horizon look bigger than they are! When high up in the sky, these objects cannot be as far as the horizon since the bowl is believed to be rather shallow!