HI Zzhangho,
You're right to want your recommender to get more into the leadership/management details in his recommendation. Top business schools are looking for people who are already marked as leadership material, and who have begun to make the transition from technical expertise to team management, big picture vision, strategy decisions, etc. And you're also right that less is not necessarily more here: most top MBA programs use the same recommender questions now, and their word limits can be seen as a suggested minimum as well as a prescribed maximum. In other words, for the question about how you rank versus your peers in certain areas, which most schools are now using, a word limit of 300 words not only means that a 400 word answer is too long, but also that 200 is too short.
It's perfectly ok for you to coach your recommender, sit down with him, with the recommendation instructions of each school in hand, and walk him through the process. The ethical stipulation is that you should not write it yourself, and that your recommender has to submit it himself, without you seeing the final draft.
How much does one letter count? It counts a LOT. EVERYTHING in your application for a top MBA program counts! Treat every task, whether it's a large one like a recommendation or one of those small text boxes in the online application, as if this is the text that's going to make the difference for you. Remember, competition for business school is fierce -- you have to be at the top of your game in absolutely every area... because your competition will be.
Good luck!
Margaret Strother