Richard Ferguson wrote:
Every boat has a designed load range and an optimum load range, which is
the engineering view of the world. I have a personal story.

I one time tried to use a friend's sit-on-top kayak. Now I am a big
guy, around 200 pounds, and heavier than he is. I found that if I took
my feet out of the water and put them on the deck, I went over almost
instantly, in calm water. With my feet and lower legs in the water, the
boat sat higher in the water, and was reasonably stable. However much
my legs weighed, maybe 30 pounds (15 kg), it was the difference between
stability and instability. After swimming a few times, and failing to
get far from the dock, I gave him back his kayak.

Your issue was not likely one of absolute weight, but center of gravity.
The only way one can be too heavy for a kayak is if you literally push
it under water. However, the higher your center of gravity, the less
stable the boat will be. Also, the heavier you are, the more effect you
will have when you shift your weight or lean. People who are tall and
heavy will find a given boat to be much less stable than a person who's
short and light.

I think that the extra weight took away the initial stability that the
kayak was intended to have. My guess is that the boat was designed for
smaller people, but usable for medium sized people. My weight was
outside the operating range of that design, making the boat unusable by
me.

That's highly unlikely. With most kayaks, the stability increases as you
push the boat deeper into the water, up to the point that the gunwales
submerge.

I would make a guess that overloading a boat is generally much more
detrimental than underloading a boat. A large boat paddled by a small
person probably will be slower and more subject to wind and wave than a
smaller boat paddled by the same person, but probably still safe and
usable.

Actually, the opposite is true. A heavier person pushes a boat down
farther in the water, increasing the length of the waterline. While this
adds more surface friction, it also increases the theoretical hull speed
of the boat, making it possible to paddle it faster before "hitting
the wall", so to speak.

I paddle low volume boats and am at the upper end of the recommended
weight range for all of them. I also build skin-on-frame boats that are
even lower in volume. All of my boats perform well and have the added
benefits of a better fit and less windage.