"We were convinced that the 16.5 rear would work but getting to the stage where you can actually show the teams the improvement is not always so easy. We got lucky with the rear because Garry McCoy was the first rider to like it and use it and won the first time out in 2000 at Welkom, everybody then thought that it must be the way forwards if Garry was winning."
http://www.motogp.com/en/news/2003/12/22/michelin-explain-the-reasons-for-16-5-front-tyre-experimentation/135689