* The sun is a star: It's a giant ball of hot gas, much larger than the Earth.

* The sky isn't a physical thing: It's just the appearance of the atmosphere above us.

So, the sun is definitely bigger than the Earth's atmosphere. But, it's not bigger than the entire concept of "sky," which is just a way of describing what we see.

Think of it this way: the sky is like a big empty room, and the sun is like a small, bright lightbulb hanging in that room. The lightbulb is much smaller than the room, but it illuminates the whole thing.