When the eye need to focus on any distance object, then in that case the ciliary muscles of the eye get fully relaxed and this also pull the ligaments attached to the eye-lens tightly. During this process the eye lens get stretched and the eye-lens becomes thin.

When the eye has to focus on the nearby objects, then in that case the ciliary muscles of the eye has to contract and make the suspensory ligaments loose. Which results in the ligaments to stop pulling the eye lens. The eye lens bulges under its own elasticity and hence the eye lens got thick.