Judo was conceived and the rules were first systematized in the late 1880's in Japan, a newly-industrializing agrarian society with a great degree of homogeneity in the population. As such, there was, as I understand it, no concept of weight classes. Everybody in the early Kodokan fought anybody/everybody else they could learn from. This potential variability in opponent size seems almost certainly responsible for the development of the judo ethic of Maximum Efficiency with Minimal Effort. Kano, in his lectures, even used the example of a man with '4 units' of strength opposed by a man with '10 units of strength.'
So, I've heard various people express their opinion that the introduction of weight classes in judo competition is greatly responsible for the perceived decline of judo from a technical height in the 1950's. In a recent poll here on Mokuren Dojo, with n=8 voting (shame on y'all! I'm sure that more than 8 people have opinions on this...), 25% described weight classes as a "just plain bad" idea. Certainly, as an ideal, one would like to refine one's judo to the point that size is simply not a factor. One strives toward that perfection of technique in which an opponent of any size is manageable. But as a martial sport, particularly in national and Olympic arenas, weight classes were eventually deemed a necessity. Beginning in the 1950's, people began implementing weight classes:
The first weight category system in Judo was developed by the Northern California Judo Technical Committee under the guidance of Henry Stone (1948). We established four weight divisions (130, 150, 180 and Unlimited). To preserve the "little man versus the big man" theory, the Grand Championships were established where the weight system was eliminated and the skills of true judo can further exemplify the principle of "Maximum Efficiency with Minimum of Effort", Dr. Kano's established "All Pervading Principle of Judo."
The weight category system and the tournament rules were patterned from our Olympic Wrestling Tournament Scoring System. All this unfolded from an old copy of the first competitive judo rules and weight system developed by R. H. "Pop" Moore Sr. at the request of Dr. Jigoro Kano during the Xth Olympic Games at Los Angeles in 1932. "Pop" was Japan’s first Olympic Wrestling Team coach. Dr. Kano envisioned the need for a weight system that far back, particularly, he was impressed with the conduct of the Olympic Wrestling Competitions, (scoring, rules, and weight categories, etc.). Incidentally, Hatta and Kotani were members of Japan’s Olympic Wrestling Team at that time.
After further experience was acquired in the use of the weight category system, it was revised to its present categories, (139, 154, 176, 205, and Unlimited). Geesink's prowess was not solely responsible for weight categories coming about but no doubt he greatly amplified this need which was already begun in the early days. His consistency in defeating top Japanese judoists in competitions in Japan and the World Tournaments certainly helped and opened up further thinking of individuals who were yet reluctant to accept weight divisions.
(from a Letter from Emilio Bruno to Don Sayenga June 20, 1972 )
The 2006 AAU Rule book describes it in terms of competitor safety:
Contrary to "popular myth", size is as important to judo as it is to other sports such as football, basketball, boxing or wrestling. Although a highly skilled small person has been known to defeat a larger less skilled opponent, the usual result historically has been cracked ribs and other assorted injuries to the smaller contestant. Since judo is a size factor sport, weight divisions should be established to minimize weight becoming the prime determinant of the winner. The days of Open divisions (all weights together) should be a thing of the past. Local and regional events may establish their own adult weight divisions, but to maximize safety and fairness. WEIGHT DIFFERENCES BETWEEN INDIVIDUAL COMPETITORS IN ANY DIVISION SHOULD SELDOM EXCEED 10 PERCENT.
There are even weight classes in Sumo now, though 'lightweight' is 187lb and below!
In my opinion, (and I am very much a non-competitive, 'maximum efficiency' type guy) I think the idea of weight classes has not gone far enough. The basic idea behind the weight classes was to control for variables so that skill and not sheer strength was the main determinant of the match. Here's you a thing to think about: How is it a controlled competitive environment when a yellow belt novice can compete in a tourney against a rokudan - so long as they are the same weight? How is that a fair test of skill? Now, admittedly, by the time you get to nationals, internationals, or Olympics, everyone is black belt, but in the grassroots of the system there is an immense inherent unfairness. I'd like to see competitions control for both weight and time in grade.
But then, again, I can also see the point of view of the technical purists. One must, of course practice and randori against everyone, striving always for that pure ideal of maximum efficient use of power, but weight classes for the more dangerous, uncontrolled shiai are definitely a good idea.