Basically, boats are designed with a water-line threshold. Additionally, performance integrated rails and chines come into play. The general weight range should consider how much weight (gross: paddler and gear) begins to engage its performance and how much surpasses it.
With the possible exception of creeks boats, most boats have an optimum range perhaps somewhere in the middle. Whereas, balanced volume boats like creek boats may afford capabilities into the upper limits. Obtaining the right size boat for a person's weight and its performance design, and then adjusting the seat to properly trim it for the sweet spot, should be critical to enjoying ones boat.
I know that it took me a long time of fiddling with my Diesel 65 before I ultimately fell in love with the boat. At times I experimented with seat, bulkhead, and thigh hooks so much that I hated the boat and boated like crap in it. Ultimately, because of its design characteristics, I found its sweet spot for my weight and body structure (short, scrouny legs, and thick chested, muscley, top heavy). Men and womens physiques and each persons height (length of legs leveraged out front) and the amount of gear that you plan to haul and the boat design all contribute to what to consider in selecting the appropriate boat.
No risk, no reward. It is not that we have to, it is that we get to. Preparation and education are essential to self-confidence and success. - KV
"If there is no risk there is no adventure."- Bill Briggs