Minecraft wiki entry for Sun, associated text: The sun is visibly larger during sunrise and sunset.
If the player is underwater or underground and looks at the sun, it appears the same as the sun at sunset. This is due to the way the sun is rendered when exposed to the sky.
Standing or flying at a height above the terrain of approximately 1.4× the render distance, the sun appears opposite to the moon in the sky, making it seem almost as if the player is in space.