Found this earlier: Landing Page Optimization: Conversion increased 37% by reducing copy not sure how accurate it is but it was interesting to read.
There are many case studies like this, but I doubt the statistical significance of the results, which they also admit. Probably the trend will be the same if they would get more clicks, and the variation is better than the original after all.
The shorter copy works when the visitors have enough info before hand, enough trust or a general idea of what the thing is about and they want to proceed further faster.