Are there any Venus Flytrap cultivars which have massive fist sized traps?

I'm afraid not. That is the stuff of Hollywood legend, possibly hyped up by the cult movie 'Little Shop Of Horrors'. Flytrap's evolved to capture and digest flies and that is why they grow to the size they do. No amount of hormone injection, steroids, fertilizers or any other additive will get traps to grow to that size.