Question

why does the size of the object appears to be changed as it is observed from different distances?

Answer 1

The actual size of an object which we see is always constant. It doesn't change whether it is near or far away from us. But the size we perceive is not the actual size of the object, it is the apparent size which depends on the visual angle.

Visual angle is the angle subtended by an object at your eye.
- As the distance of the object increases, the visual angle decreases and the object looks smaller.
- As the distance of the object decreases, the visual angle increases and the object looks larger.