Why we can see the sun bigger in the early morning/evening and smaller in noon time?

It's perspective that makes the sun (and moon) appear larger when it's on the horizon. When you're watching the sun creep up over buildings, trees and the like, it does tend to appear much larger, because of the perspective of size imposed against static points of reference that your brain has programmed over years of experience. When you see a large building, your mind has an experiential point of reference of how large it actually is, but if that large building were the same distance away from you and hovering in the middle of the sky, without any points of reference, it would seem much smaller than it does standing on the ground next to other buildings. Basically, it's an optical illusion.  more