You can only see things within a certain angle in front of your eyes. If something is really close to you then it takes up that whole angle and so looks really big. If something is really far away then it only takes up a small bit of that angle, and so looks really small.
There’s a game you can play where you try to guess how big things are. The game makes you think about how big something actually is and how far away it is. It’s a good game for teaching you about angles and sizes and how we see things. You can try it any time, just pick something you can see and then try to work out how big it is (so you could look at the door of a big church and say “I think the door is 4 metres high, because adults are about 2 metres tall, and the door is twice the hight of that adult who is standing in front of the door”). Have fun! 🙂
Because they’re a long way away from you. Everything will look smaller when it’s further away from you.
Also when planes are in the sky you don’t really have many objects around it to compare it to, so it’s very hard to judge its size. This is why the moon and the sun look bigger when they are near the horizon than when they are in the sky. They haven’t actually changed in size, it’s just that near the horizon we have buildings and trees to compare them too, and we realise how huge they are.