As a layman in martial arts, I've been curious about this question: In a real life situation, is it easy/difficult/impossible for a seasoned professional fighting expert (in whatever discipline of martial arts you name) to beat a much stronger man, without fighting training but superor both in size and strength to him? For the latter think of those husky men pulling trucks or hurling ironballs in world-strong-man competitions.
"Real Life Situations"
There are too many variables encoded in this term for it to have any meaning. Is the big man stealing your car? Running you over with a car? Picking a fight at the bar because his wife left him and he's sad and drunk? Throwing one ill-conceived haymaker? The problem with such questions is that the premise of "highly trained and conditioned fighter versus individual of enormous physical capabilities" is inherently chaotic even before one adds situational concerns such as who decides it's a fight first.
It sounds like what you're trying to ask is, "without odd sport-specific restrictions, how much better at fighting bigger, stronger opponents does learning how to fight make a person?" We can find suggestive parts of the answer in a few areas.
Areas of Evidence
- Early MMA: in the 1990s in America, mixed martial arts had no weight classes. In the early days of the UFC, Royce Gracie finished a long succession of fighters who had a large physical advantage but a mere fraction of his jiu-jitsu experience. Eventually, as jiu-jitsu skill and tactics became a more level playing field, smaller men had a tougher time beating bigger opponents. It still happens, though: Anderson Silva, the 185-pound division champion, has defeated several 205-pounders in dramatic fashion.
MMA mismatches: Japanese mixed-martial-arts promotions had a liking for circus fights. That is, a smaller but more skilled man (often quite large in his own right) would be pitted against a GIANT. (For example, Bob Sapp versus Nogueira, or Fedor Emelianenko versus Hong Man Choi, or any of Fedor's other fights against enormous men.) Alternatively, a hard-hitting man with an iron chin but not much training would be pitted against similar-sized men with more experience (e.g. Mark Hunt).
The successes (and failures) of brawlers in MMA is quite informative on this topic. Tank Abott and Kimbo Slice are good examples.
Absolute divisions: Kyokushin karate, judo, and Brazilian jiu-jitsu retain no-weight-class divisions. These are full-contact striking, grappling, or striking-and-grappling contests, and can thus be informative about what works and doesn't work against bigger, stronger opponents. (Light-contact karate tournaments are not so informative, because not being able to clinch, grab, or hit hard removes a significant portion of the benefit of size and strength.)
Marcelo Garcia is a great example of a smaller man successful in grappling against bigger opponents. This highlight video shows him demolishing grapplers who are often bigger and nearly always stronger than him. These opponents are no slouches, either; many of them are champions in their own right. But Marcelo's victory through technical superiority and innovation is not absolute: he lost against Roger Gracie and Jacare Souza, both of whom possess elite-level fighting skill as well as significant strength and size advantages over Marcelo.
- Street fight videos: we can condemn the stupidity of street fights while pumping videos of them for knowledge. One notices when a street fight involves one or more persons with combat sport training: the clinch is still messy, many of the punches may still be haymakers, but techniques get applied to great effect. Combat sports with a strong emphasis on hard sparring tend to have more and better clear examples of their techniques used successfully in street fights. For instance: boxing, boxing again, BJJ (featuring a champion against a somewhat bigger and perhaps stronger but definitely drunker opponent), and judo. Finding examples with a strength/size mismatch is much tougher; but we can extrapolate that the results are similar but less successful.
In these combative arenas, smaller and weaker fighters frequently best stronger, bigger opponents. They frequently lose, too. Technique matters, but if technique is equal then physical attributes (size, strength, agility, power, reach, speed and so on) matter a lot. The standard advice for beating a physically superior opponent is to have superior tactics, technique, conditioning, and speed. In comparing combat sports to unscheduled street fights, some of those areas may be tougher to bring to bear.
It really comes down to whoever is better able to apply their strategy in the fight. If a champion can tire out their opponent, getting kneed in the face by the champ will not help their ability to fight back.
However no matter how skilled you are, in a fight you will get hit. Over a series of fights these add up. So at some point a champ might get knocked out relatively easily.
The fact fighters cut weight to get into lower weight classes implies it's worth having the size advantage.
In a self-defense scenario, between intelligence and physical skills, the latter gives you advantages over a untrained person, whereas the former gives you more chances of 'surviving' a less-smart person than you. If an untrained person is smarter than you, no matter how much you have trained, you will end up with a bottle on your head before you realize it, and thus all your physical training will be wasted.
I love MA, and I train it for many different reasons, but if I had to choose between being really fit and well trained, vs being really smart, if we are talking about self-defense, I would choose the latter. MA training will give you the confidence, and the speed, performance, etc. But in many cases it will not directly address matters such as your emotional state (before, during and after the fight) or train you to 'cheat', or to use other techniques in order to be 'smarter' than your aggressor.
In some types of aggression, you can often spot the Point of No Return, that is, you know that this guy is going for you and there is nothing you can do to convince him not to crush your skull. His body language will tell you that: his upper lip will be curled up, his chest will be forward, his breathing pattern will be more accelerated, fists clenched up. At this point, you need to acknowledge that you will 'die', unless you do something about it. So you hit first, you use a weapon, you distract him and run, you offer money to someone to help, whatever, as long as you get him by surprise. The goal on the streets is not to compare skill vs strength, but to survive by whatever means you have: usually intelligence will get you further.
To sum up, you don't need to necessarily train MA in order to really gain advantages against an aggressor on the streets: you just need to be smarter than him (and that also means, realizing that someone is starring at you and that you should leave the scene before things escalate further). If contemporary MA schools started to adopt this 'Urban Survival Combat' kind of thing and started getting people to realize the importance of outwitting an aggressor, then I would change my answer to say that: 'Yes, MA really train you for the streets'. Unfortunately most of MA disciplines have moved from a discipline that was to teach the individual to fight as part of an army to a system that train people for sports. Neither are suitable for a street self-defense where it is not very likely that you will be accompanied by a small army with you or that the aggressor will say: 'I will crush your skull using Karate rules only'.
If, however, you are a good MA student, smart and capable of not letting your testosterone mess up with your brain, you learn how to avoid this situations and should you ever get into them, you will know how to defend yourself before you attacker has even the chance to blink. So my answer is: 'Yes, MA will help you a lot, but what is covered in a MA school is only a tip of the iceberg on your self-defense study, and you will need to learn a lot about the psychology of different types of attacks/fights and tame your emotions'.
Some Reading on the subject: