Its easy to turn 16,000 with larger engines that are multi cylinder because you have the other cylinders to counter balance the opposite cylinder in the stroke cycle. It gets a lot more difficult with a single cylinder engine above a 62-66mm stroke.
Its also about weight of components in the 4 stroke valve train and pressures needed to keep them following the cam profile to reduce valve bounce, you cant just keep going up in spring pressure. I don't think the valves in a 4 cylinder 600cc bike engine are the same size as a 450-500cc single.
