I suppose part of the answer is that the smaller monster didn't quite manage to actually eat the bigger one. I mean, it certainly had a go; it managed to swallow it and get it into a stomach that has stretched so much it's gone transparent, but the digestion process doesn't seem to have gone too well.
You might have an inkling as to why this creature is called a Black Swallower. In this case it has swallowed something so big that it started to decompose right there in the Swallower's guts, releasing gases that meant they both began to float to the surface.
It must have been quite a journey; Black Swallowers inhabit depths between 700 and 2,745 metres (2,300-9,000 ft) in tropical and subtropical oceans around the world. The biggest found was 25 cm (10 in) long. They eat other fish, or try to, and can clearly swallow rivals bigger and heavier than themselves. It's thought that they grab prey by the tail and inch forward, their stomach growing increasingly grotesque as it fills. Next time you put your socks on, imagine it's a gruesome fish slowly eating you feet first and you'll get the idea.
I wonder if it could make for a good motivational poster for dieters?