that's why you should hit him in the back of the head with a chair, so he won't injure you. One big guy can't take down that many small guys, it ain't happening. If no one thought like that (being scared of getting hurt themselves) that poor guy who got beat up would only have had to suffer that first punch.
You're making it sound as if it's easy to beat up a 300+ pounds person. It's not. I don't know if you ever been in a fight, but it's quite easy to die from it, especially if it's a 300+ pounds person.
The very last thing on my mind would be to challenge a 300+ pounds person on purpose. Despite what you may think, fighting a person has certain risks that may or may not be permanent. If I knew for certain that if I got in a fight, that I would walk away with only cuts and bruises, I would be busting heads and breaking chairs over people. But it's not. Real life doesn't work like movies. Making a single error can result in your life.
Acting like a vigilante is silly. If you see someone getting beat up, you don't get your bats and start breaking skulls. Police recommends the following: note the appearance of the individual(s), call 911 from a safe place, try to get the general direction the persons went, and simply wait for the police so you can make the statement.
We live in an age of camera cell phones. The smartest thing to do to help the person would be to take a picture or short video clip, and when the police come, hand over the picture/video so they can make a report and possible an identification.
Not only that, a 300+ pounds person can easily damage a person. The person would need medical help and getting in a fight can cause more harm to the person.
If you just start punching people and screaming, "Let's get the party started!!!" And then start tossing hot pizzas and busting skulls, you're actually doing the person you're trying to help more harm than good.
It sounds good on paper, yes. But realistically, you're not going to beat a 300+ pounds person without taking on some serious damage.
It seems like the common assumption is that people are suppose to bend down besides injuried people and start wailing and pulling hair out of their head. Come on. Don't be silly. Just because you stand around, it doesn't mean you're not helping. I mean like Calcium said, there is only so much you can do when someone is down.
The best thing to do is call for help right away.