Why do the sun and moon look huge on the horizon?

When viewing astronomical objects, you are looking through an ocean of air. This air refracts light and makes it s-p-r-e-a-d. An object low on the horizon has much more air between it and you, as the observer. Another factor is that an object will look larger when other reference points are present to compare it to (such as a building, tree, or house).

Just for fun, try this . . .

Hold a coin between two fingers of the same hand at arm's length. Raise your hand so that nothing but open sky is behind it. Stare at it intently.

Now . . .

Watch the coin closely as you lower your hand from the openness of the sky to the horizon where other objects can now be seen in your line of vision, too. As other objects appear, the coin begins to look as if it is getting larger.