The sun doesn't actually change size throughout the day. The reason it appears bigger in the morning and evening is an optical illusion caused by a phenomenon called atmospheric refraction.

Here's how it works:

1. Light bending: As sunlight travels through the Earth's atmosphere, it bends due to changes in air density. This bending is more pronounced when the sun is low on the horizon, near sunrise or sunset.

2. Apparent position: The bending of light makes the sun appear slightly higher in the sky than it actually is. This illusion makes the sun look larger, especially when it's near the horizon.

3. Perspective: When the sun is low in the sky, it appears to be closer to objects on the horizon, like trees or buildings. This further enhances the illusion of size.

In summary: It's the bending of light through the atmosphere and the way we perceive size in relation to other objects that make the sun seem bigger at sunrise and sunset. It's not a real change in the sun's actual size.