Answer
Lenses made with higher index of refraction material are relatively thin.
Work Step by Step
If the eyeglasses are made with "high index of refraction" glass, refraction is stronger at the surfaces when light enters or exits the glass from air. This means the light bends more, the curvature of the lenses can be less severe, and the lenses can be made thinner.
This is discussed on page 526.
