"But when you see a person what makes you think 'they are fat'?() "
Not sure how you can usually tell if someone cannot see their dick, unless they are really fat. I honestly don't know how fat someone should be at the least to not be able to see it.
Size is one of the factors, I can't really think of another. Someone being merely overweight at 5FT tall would look, as said above, really round. It's just with their short height they do tend to look fatter. If someone was 7FT and overweight, they wouldn't really look fat. It would more look like they have a big build. As also said above, it depends on your build.
Personally being too skinny is when your elbows are really sharp and the bone sticks out, it isn't always the case though. Not to mention with most parts of your body, such as your spine sticking out without even trying. Basically it's when your bone starts sticking out and makes you look less attractive, at least to me.
I disagree with this. Moobs, I've seen people that have larger moobs than others, I find it weird and unattractive, some even skinnier than others. As mentioned above, not being able to run for long periods of time could be caused from asthma or even smoking (Plays a role at least). Not to mention at the gym, I saw this overweight guy running for a long time. Don't really wish to argue with the gut/belly over one's belt, having a big gut can be inherited and the only thing that would really be bigger than normal is the stomach. But you need to know that it isn't "fat".