"Big Nothing" Is a lot of work effectively for nothing.
A good example would be of someone who is depressed
and their view on life.
Or being given a task or job which the effort needed is far greater than the rewards are worth.
Depressive would say: "Life, is all a big nothing".
A classic example would be;
"If you eat 100 nails, live for 1000 years and do it by doing the most depressing mediocre way possible I will let you have this jar of air"