Chiasmodon niger has long jaws with recurved teeth that walk prey backwards into the throat, and a stomach wall thin enough to distend enormously. The ribs do not enclose the abdomen in a fixed cage, so a swallowed fish can protrude below the body as a visible bulge.
Specimens have been recovered containing prey far heavier than the swallower itself, including a much-cited case in which a black swallower found off the Cayman Islands held a snake mackerel several times its own length. The swallower in that case was dead and floating, and this is a recurring pattern: prey too large to digest quickly decomposes, gas builds up in the stomach, and the fish is carried to the surface.
That means the specimens most often photographed represent failures rather than typical meals. How frequently black swallowers take prey they can actually process is unknown, since almost all observations come from animals that misjudged.