The problem

Fat bulging in the lower eyelid that gives the appearance of puffiness or a bag. There may be one big pouch, a single small pouch, or several discrete pouches. Shadowing or a dark circle may be present below the fat bag.

Causes

There may be an extra amount of orbital fat (the fat surrounding the eyeball) that bulges forward.  Alternatively, the areas beneath the eyelid at the upper cheek may have become more hollow or sunken, and this can accentuate the apparent fullness of the lower eyelid. The area beneath the inner corner of the eye, sometimes called the tear trough or nasojugal fold is the most common place to observe a depression or a valley.  It is for this reason that the area just above this on the eyelid is the most common place to see a bulging fat pad.  Sometimes there is truly bulging fat, sometimes there is only the illusion of bulging fat because of a valley, and sometimes both are occurring at the same time.

In the photos below, you can see that if the hollowing is eliminated with photoshop, the bulging is no longer visible. This illustrates that lower eyelid bags are often secondary to volume loss.
lower steatoblepharonlower steatoblepharon corrected

Often seen with

Sagging lower eyelid skin or lower dermatochalasis, bulging fat of the upper eyelid or upper steatoblepharon, loss of facial fat, and lower eyelid hollowing.

Not to be confused with

Malar mounds or festoons, eyelid edema or swelling, loose lower eyelid skin or lower dermatochalasis.

What you see in the mirror

Puffiness, pouches, or bags under the eyes, possibly with sunken or hollow areas beneath the bags.

dr. brett kotlus eyelid fat bulging

Bulging lower eyelid fat (lower steatoblepharon)