Ptosis surgery and upper eyelid blepharoplasty are often combined in order to (1) correct the droopy muscle and (2) remove fat and excess skin.
Photos of patients who have undergone eyelid surgery for droopy upper eyelid (ptosis) combined with removal of excess herniated fat, baggy eyelid - upper blepharoplasty
The animation below illustrates the scenario when there the patient has ptosis BUT ALSO has with some amount of excess upper eyelid skin
As the eyelid ptosis is corrected (the muscle is adjusted to lift the eyelid), the edge of the eyelid and the upper eyelid skin move closer together.
If there is excess upper eyelid skin, once the ptosis is corrected, the skin my OVERHANG the edge of the eyelid: Slide the control to the right to see a demonstration of this effect.
As the eyelid is elevated, the skin that is present on the upper eyelid, bunches up (accordion effect). The upper eyelid skin might roll over the edge of the eyelid); slide control to the far right to see this effect.