Why is it that at sunset the sun seems to be bigger than at mid day?

It is known as the "Horizon Effect." The moon also looks larger when it is just rising. It is an optical illusion because when the sun or moon are on the horizon, you have landmarks as a reference point, and the size is exaggerated by the fact that it looks like the object spans two distant mountain peaks or whatever. But once the object rises, you have no point of comparison anymore. A simple test reveals the optical illusion. Hold a dime or quarter in front of your eyes so that it just covers the sun or moon. As it rises, hold that coin at the same distance from your eyes. There is no difference. Suggest trying this with the moon, for obvious reasons. You don't want to stare at the sun.

Careful measurements will show you that it is the same. It's not really an optical illusion, it just looks like one. Both the Sun and the Moon are closer to you when they are over head rather than setting or rising. Go figure.

I don't think it's brighter, the light is just more diffuse. Er, spread out over the dayside of Earth more since the source is fading and less intense.