Raptor,
Without trying to put words in Brandon's mouth, I feel that the 1/3 recommendation is due to the swing almost being more of a plyometric/low inertia training modality, rather than a "lift". Obviously an athlete who is incredibly strong can use more weight, but the book "easy strength" where I found a lot of my original material talks about many athletes who have found great results without heavy bells. Donnie Thompson, who has the WR in powerlifting in the superheavy and squats well over 1,000lbs "only" uses a 100lb bell for his training.... which is just under 1/3 of his bodyweight I believe.
Obviously the 1/3 bodyweight bell is a general recommendation, but I would imagine for the majority of trained athletes who squat around 1.5-2.0x bodyweight and SVJ 25-35" it is a solid solution. Not to mention, even if you are incredibly strong, the swing is all about speed! Even if you squat 3x bw, you can't turn that strength on quickly enough to reverse the bell faster than someone who squat 2.5x or 2.0x bodyweight.... providing it is a light bell.
An anecdote was given in the book about a guy who was 5'9 and had a 2.5x bw squat... maybe more, who went from grabbing rim to dunking by using a 50lb bell I believe? The weight doesn't have to be high... of course, stronger people can use a bit more weight... maybe 1/2 maximum I would say, while weaker athletes would probably want to go more around 1/4... but I think the 1/3 range is where most athletes can benefit the most.
Finally, that weight... 1/3 was the "sweet spot" where most athletes could produce the highest force on the force plate.
Joel