Cetaceans like dolphins have several stomach chambers, starting with a muscular "forestomach" that acts like a crop to crush and masticate food. This is of limited size, and so despite their impressive size and gape most such creatures can only manage comparatively small prey.
Occasionally a cetacean manages to push a food item past the forestomach into the much larger main stomach, which is very uncomfortable the first time they try it. If they do manage it, the next logical step is to stuff the gullet full of food as well, resulting in the occasional Stregaverse dolphin swimming around with another dolphin's tail hanging out of its mouth. Huge prey like this is digested bit by bit as room is made in the main stomach.
Like competitive eating among humans this process is very wasteful, the stomach and intestines simply cannot process and absorb this many calories all at once. Many of the calories are lost and simply passed unabsorbed.
The fact that this whole process takes a lot of getting used to and is wasteful is of little comfort to the prey, who at least are usually swallowed headfirst and are spared being slowly digested from the feet/tail on up. 83