The weight range of a boat is as dependent on design as on volume. Modern playboats, for instance, pack a lot of volume in the center so you can spin around your center of gravity, regardless of direction. Creek boats have length for speed, rocker for turning, and features that influence tracking and turning. They vary with design.
I'd trust manufacturers recommendations for the most part, because most of them test extensively to figure out what works for most people. Prijon has a long reputation for getting those things right. They blow mold, which is much more expensive than rotomolding (the usual process), so it's in their best interests to get it right. It takes several hundred boats for them to break even on a mold.