That's why you have to make sure you have a good contract with all potential what-ifs covered. Then the big company will have to find a what-if that you didn't cover and if you're luck it will be ridiculous enough that you won't need a massive legal team to defend your agreement.

Either that or you go into it just hoping to get all of the money, but you make sure that the up-front money is enough to keep you floating down the river.