Very interesting topic and I would think we are not far off a minimal width requirement somewhere in the future. It makes sense as we have divisions for length - 12'6, 14' and unlimited and eventually there would be a minimal width to at least allow the manufacturers to work to a template range.
As far as length goes, we had a 500m sprint here where we weight restricted participants to 12'6 and 14' boards. <80kg went on 12'6 boards and raced each other - the results were interesting and it kept the lighter guys together. Over a sprint this is where the advantage of being light is limited.
Over distance, I'm not so sure. Marcus is dominant over here and he weighs 65kg, but he trains his butt off (he does 7-8 sessions a week). I'm 88kg and train regularly and can match most of the lighter guys that train the same as me.
Perhaps it is a case of training harder to go faster?