Question

A lemon in a glass of water appears bigger and not smaller, why?

Answer 1

The lemon appears to be bigger when in a glass of water.This is due to the phenomenon,REFRACTION.

When light passes from denser medium to rare medium i.e.,from water to air, the light refracts away from the normal.Hence the lemon appears bigger.

                                 
                                         Hope it helps..............

Answer 2

It is due to refraction of light.
     It appears bigger as when the light rays enter water, they undergo mutiple refractions causing the object to appear larger.
  I hope it helps you.