With regard to self-defense and use of deadly force laws in the various states of the US, most boil down to the same basic elements. They're pretty simple.
1) You have the right to defend yourself against unlawful assault by another person.
2) You may have the right to use deadly force to defend yourself (in some states, also your property, home, or others) if a reasonable and prudent person would believe that they were in imminent danger of losing their life or suffering grave bodily injury.
Now, as I've said, I'm not a lawyer. This is not legal advice. But my *opinion* is that although the above is pretty simple, it's when we get down into the weeds that people seem to lose the thread.
There are no hard and fast rules about what a reasonable and prudent person would believe for a written-out list of various things which might happen to you. "Well, you see, if he's a big guy and he has tattoos and has been known to fight with others and his brother's name is Larry and it's the 3rd Tuesday in the month, then yes, you may draw your legal and licensed concealed sidearm and make his head look like a canoe."
What happens is the police investigate and take statements from witnesses, gather evidence, and it gets turned over to detectives and/or prosecuting attorneys. The DA may decide that the incident is a clear-cut case of self-defense and choose not to pursue it. Or the DA may convene a Grand Jury and put the question to them whether or not to indict. And in either case, if the DA prosecutes, then the question of 'reasonableness' becomes a question for the judge or a jury depending on the type of trial. The defense will attempt to prove that the shooting was 'reasonable' and the prosecution will attempt to prove it was not. The prosecution has to prove their case, the defense does not have to prove innocence; they only have to be able to cast 'reasonable doubt' (there's that word again, different context) on whether or not the shooter is guilty.
The jury can consider all kinds of things, up to and including, yes, disparity of force. But here is where people tend to get lost, IMHO. There is no 'disparity of force' statute that I am aware of, anywhere. It's a jury instruction. It's an argument that will be raised by the defense or the prosecution (whichever it tends to favor in any given circumstance). It is not a 'rule'. It is not a 'law'. It's something to be considered during a trial and potentially during jury deliberations.
This is why people like Massad Ayoob write about such things; because they can matter to a judge or jury. It's something that anyone who goes about armed should understand; when they are and when they are not allowed to use deadly force to defend themselves.
A jury can do all kinds of funny things. One day, a jury can decide that a 130 pound many who was shot was reasonably killed because the 260 pound man who shot him was in fact in reasonable fear of his life, despite the disparity in size. Another day, another jury, and they might come to the opposite conclusion.
This is what happens in courtrooms all across the US. The law tries to remove arbitrariness from the results of criminal legal proceedings, but there is no way to do that. Bad lawyers, bad defendants, bad juries, even bad judges can all make what seems like an open-and-shut case of self-defense into a serious crime - or vice-versa.
None of this bothers me overmuch. What I guess gets up my sleeve is when someone reads something and decides that's 'what the law is'. No, it is not what the law is. The law is simple. People fitting the law to actual events in a courtroom, that's hard. But there are no hard-and-fast rules for how that is going to end in any given case.