I don't think the editor will assign the referees a different version of the paper; after all, they are supposed to be judging whether the journal should publish the paper they are reviewing or not. I think you could post an extended version on the arXiv and make sure the referees are aware of its existence. Some of them may then give you useful feedback about whether they found the extended version helpful and may suggest moving some material into the main paper or vice versa.

Having said that, I think your chances of having the paper accepted are improved by submitting the best version you can up front. If you're not sure whether your proofs are too short or too long, have someone (your advisor?) read the paper and give you comments before submitting. This can only help your chances.