Refraction is the change in the direction of the wave passing from one medium to another.
The change in the direction of the propagation of any wave is due to the different speed at different points.
A far away object appears closer and a small object appears larger.
Take an example. Suppose, an object lies in water.
A far away object appears closer. The angle from which the rays of light reach the observer is larger than the angle that it makes in air.
This makes the angular size larger to the observer's eye which makes the object look larger relative what it looks in air. Similar but vice versa condition occurs in the case when a small object appears larger.
