Many martial arts as well as practical methods such as Krav Maga claims to have the answer for difference in weight and size. However, even in competitions the experts have to be in the same weight. I would like to know if there are any practical techniques that can be used in street for overcoming difference of strength, size and weight. I ask about techniques for self defense not for tournament.
Your size and strength is relative. Skill level matters more, or the ability to acquire the skill and the skill of the opponent. Check out this you tube channel here , this guy is really good at explaining the concept of energy and power and it can be applied to multiple martial arts.
Ultimately, its better to know a few techniques very well, instead of knowing many techniques poorly. Usually techniques that attack the joints or soft tissues, and techniques that use the opponents force are more effective against larger opponents.
As of particular techniques for street self defense, it depend on the type of attack but USUALLY there's always a weak spot, and most systems teach to defend against common attacks.
If you're in situation were you need to defend yourself, your goal should be to scape safely with/stop the attacker, so places to attack would be Soft targets like: eyes, throat, fingers, groin, inner thighs, and toes. Impairing vision, ability to walk, ability to grab you are more useful in escaping safely if you cannot stop the attacker.
The trick is to move inside their range. Then it's a case of the bigger opponent having to fend off the smaller one because you're no longer in their striking range. Bruce Lee was a short fellow, and he used a combination of parries to move into range, then do things like backfist, rutts, uppercuts, tight hooks, overhead hook to get the job done.
in competition a while ago there was Mike Zambidis who used to take out opponents much taller than him by slip-counter type moves . The Lethwei fighters do this too. (get close in range and pound)
If weapons are available, these quickly equalize situations, but I suspect you are interested in unarmed situations.
There are no best techniques that always work. Any decent martial arts system will have techniques that do not rely on you being bigger and stronger than your opponent. These should work for you provided you have sufficiently trained.
Wong Kiew Kit describes the relationship between technique and force in this excerpt from The Art of Shaolin Kung Fu, pp 134-135:
In a fight a seven-year-old black belt, for example, will be no match for a powerful adult, even though the child is trained in martial arts and the adult may have no knowledge of martial arts at all... The decisive factor here is force.
In terms of force, the relationship between a Kung Fu master and an ordinary person is like that between a powerful adult and a child.
Hence, the mark of a master lies in depth of force rather than range of techniques. Of course this does not mean that techniques are not important; it only shows that force is generally a more significant factor in deciding victory in combat. When the question of force is not relevant, as when children fight among themselves, or when students of similar force levels engage in sparring, techniques become very important.
The word 'force' is an imperfect translation of the Chinese term gong (pronounced 'kung'). The term 'Kung Fu' (gongfu in Romanized Chinese) comes from this concept of gong, indicating that the emphasis is not on learning techniques but on training force. Besides force, the concept of gong also includes other factors like skill, speed, agility, balance, fluidity of movement and correctness of form.
In developing the gong concept of force, you need training to improve reactions, get faster, minimize the distance you need to cover, increase the amount of power you generate, increase the efficiency at which you transfer that power, and so on. Basically, you have to do a lot of hard work to make your techniques effective, and you need still more work to make up for being smaller or weaker than an opponent.
In this school of thought, your technique of choice may be something as simple as a straight punch, just delivered with exceptional skill.
Your equalizers are to fight unfair: attack by surprise, use weapons that the other side doesn't have, attack with methods that harm a person in such a way the injuries impair response regardless of the long term consequences to the person you are attacking.
You'll notice that competitions remove all of these things - you're expecting a fight, you have an area to fight in, it's well lit, you're told when to start and stop, you do not get to spring surprise weapons the other side doesn't have, you don't get to gouge eyes, or stab people in the neck, etc.
Once you've removed all of these, you come down to skill, size and power... so if you want to just see skill, you have to start matching people up of similar size... that's what a competition is about.
However, if you're not trying to see who has the best skill, but rather win no matter what, then there's no such thing as cheating - just doing everything you can to make it unfairly favoring you - which is what you do in a situation involving your safety.
There's no singular technique, but most of the combative styles you can learn will focus on these things.
Honestly, if the size, mass, and/or strength disparity is wide enough overcoming such an opponent is not possible without some sort of force multiplier (e.g. a weapon).
This is also too broad a question to realistically answer. There are too many unknown factors. What are the physical discrepancies between combatants? What are both combatant's skill levels? What is the environment and circumstance of the confrontation? What are the stakes?
There is a very real hierarchy of contributing factors when it comes to combat effectiveness, and Skill level is definitely placed below Mass/Weight, Height/Reach, and Strength/Fitness. What that means is that a smaller combatant needs to be significantly more skilled/experienced than their larger/stronger opponent to compensate for the difference. Even then, sometimes the physical advantages are insurmountable.
Jiu-jitsu can be used to null-ify your oponent regardless of his size and weight for example. But if the guy knows a thing or two, it might get complicated UNLESS YOU ARE AT A VERY HIGH LEVEL! (checkout the first UFCs for examples)
Judo applies the same principle (there is a Youtube video of a 70+ year old man with a x degree black belt that can't be taken down by a strong 30 something man). Kickboxing applies the same principle (get in, hit, get out..).
The point I'm trying to make is that to overcome your 'bigger & stronger' opponent you must be extremly confident and master in your technique(s) regardless of origin and strategy.
Does this make any sense to you?