In the old "first to invent" system, BigCorp can make up document of having invent the things first, too. The only different is, with first-to-file, the "filing

In the old "first to invent" system, BigCorp can make up document of having invent the things first, too. The only different is, with first-to-file, the "filing record" is public, as opposed to the "inventing record" which does not have to be published. Also, first-to-file means that the sooner they file the patent, the sooner it'll expires. With first-to-invent BigCorp can claim to invent something for ten years and not file for patent, effectively makes their patent lasts "17 plus how many year they keep it secret without filing". BigCorp will always have more resource than individual inventors, the different is first-to-file system forces everything to be put in public record.