Can a person who's not physically strong but been trained in martial arts etc beat up a huge physically strong bodybuilder ? read below?

This is a troll. Don't feed the troll and it will  go away.

Size is a factor, skill is a factor, strength is a factor. People like to think that as long as they have skill, they’re good. This is an excuse to only work on the skills that more enjoyable to train. This way you get to go train in your martial art, tire yourself a bit, and leave knowing you did some work while convincing yourself it’s all you need to do.

The truth is that the untrained will always be better off trained. The trained will always be better off if they also train for strength and explosiveness. When skill is equal, the bigger person usually has an advantage. If you are small, every mistake you make is more dangerous than if you were bigger.

Make yourself better. We all only have so much time we can train, but train to be skilled, to be stronger, and to be more explosive. As long as you’re trying to be better that’s all you can really do.

Sure. But, just because they do martial arts doesn't mean they have plot armor lol.  This isn't an anime where knowing martial arts means you'll win. 

Strength and size are definitely factors. But they do matter less than training, reflexes, technique, and arguably "functional" strength. A bodybuilder might have large muscles. But the way they utilize them doesn't directly translate to martial arts. Not that it doesn't help of course. Size and strength definitely make up for a lack of techniques with huge disparities in size. Like +100 pounds advantages. But carrying weights is different than resisting a person. On the other hand a lot of martial artists (primarily the old school types) underrate the importance of athleticism in successful application of technique. So while many can execute techniques well they don't always have the speed/strength/agility/power/conditioning to cause the necessary damage or utilize it against much larger opponents. But it depends on a number of things. 

https://www.youtube.com/watch?v=dbpJ1pr_wG8

Bob Sapp was always a joke in MMA and Kickboxing. Even though he's losing here to someone with better technique he had a few notable wins. He weighed about 340 pounds in his prime. Here he has an over 100 pound weight advantage. Slammed Nog around, and still got subbed. 

Yes. Royce Gracie proved this in the first few UFC's

A lot depends on your definition of "not physically strong".

To do damage to a bigger opponent, one must generate some measure of force.  Punching technique, for example, won't help you if your body cannot generate enough force to tear a wet paper towel.

If you mean a person that possesses some lean strength but is not particularly tall or "bulked up" then yes, good technique can overcome a size/bulk disadvantage.

I knew a guy that was about 5'4" and average build, but he was also trained in boxing.  He got into a bar fight with a powerfully-built guy over 6 feet.  The guy tried to grapple with him but the shorter man sent him to the floor with a  perfectly-timed uppercut to the chin.

A friend who trains in Muay Thai told me about his teacher..a guy that stands about 5'6"..who sent a 6'4" college linebacker flying with a push-kick to the chest when the player hurled racist slurs at him in a bar..

But he also told me the guy's body fat was around 3% and he did 30 minute rounds on a heavy bag almost every day.... so some measure of conditioning must be there.

That is true. It really is all abt technique not strength.

When i actually test or for demos the crazy thing is they have us do them when we are already been training for over an hour and you are starting to get tired. Funny thing is the more tired you are the more technique you have to use.

That just an example in a controlled environment. I know for fact that a small guy who is trained can and has beat bigger stronger guys. It is all abt technique, timing, distance, etc...

In my case when you beat 1 guy it maybe just luck. Yo beat 2 than maybe you have some skill. You beat 3 in 1 sitting than that something you really need to look at on how and why a smaller person with some strength ad lot of skill beat 3 very physical trained men, in less than 3 minutes.

It all abt how you train and what you train in. It not just get strong and learn some boxing and wrestling. In a fight you have not even 1 second to make a decision. And it could cost you your life.

If you train i MMA and follow that training, well you just trained for sport not street. BIG DIFFERENCE in training and mentality. Nothing replaces the real training principles. Which is train as if your life depends on it.

FYI on the street there are no rules, no times out, no corner man, no one to back your 6.

U run what u brung, hope you brung enough. That saying was not just fitting for the street racing days but also for life on the street.