If someone is morbidly rotund, obese, or even a little overweight they can be described as being 'fat enough to bend light'. This derives from the fact that massive solar bodies like the sun have enough gravitational pull to alter the path of light rays, a process known as bending. Hence if you suggest someone is fat enough to bend light you are implying they have a weight equivalent to a large star, i.e. many millions of tonnes, and hence must be mocked as such.
"Golly gosh, that poor girl is fat enough to bend light!"
"Ha! Ha! Ha! Michelles husband is fat enough to bend light. Let's go give him a cake!"