Light is refracted when it passes through water into air. As light passes from one medium to another, it bends and refraction occurs. As a result two different refractive index of two medium are formed.  When one line crossed the boundary between two media, it continues to be in strait line. The direction of the same line is different than it was in the previous medium. Therefore, when viewing an object it appears to be raised and also creates magnifying effect.  So, lemon looks bigger than its actual size.