Your statement that "gaining 20kg of extra fat and keeping it" isn't healthy is entirely wrong. Everything is subjective, and health is a very, very personal definition.
I think it isn't a wrong definition but it is a little inaccurate. Having 20kg of extra fat (or even only 10) contributes to degrading your health over time. 10-20kg of fat is a serious amount and it can considerably change the chemistry of your body. In some cases (e.g.: estrogen) even a little constant change can have a dramatic effect if you have it for a prolonged period (as your hormonal system works with very tiny amounts, yet it has a terribly huge impact). And fat tissue full of veins have a huge potential to change a lot of things even if it makes each of these parameters only a little bit worse. Basically all obese people have high blood pressure as you have to feed the extra tissue. This terribly increases the chance of heart problems especially in the long run. A lot of serious diseases are hands in hand with obesity even if we are looking only at trivial statistics.
There are people who are healthy and obese people but I'm pretty sure that their health constantly degrades over time because of their extra weight. Being overweight has some other trivial disadvantages, some of your joints wear away more quickly (typically the knee joint).
You. Cannot. At all. Determine health based on looks. Is it Faaaairly safe to say that a 600 lb person has health issues? Yeah, sure. But the average citizen? You cannot make that call based on weight alone. Not for 20 kgs, or any other arbitrary number you throw in there.
I can determine overweight based on look (and I'm not talking about BMI and total weigh, I'm talking about an approximate body fat percentage - its obvious when someone has fat under her skin and after some degree of obesity the shape of the body obviously reflects obesity), and overweight is something that slowly but inevitably ruins the body. Maybe the individual has gained it only recently as a result of laziness (or whatever), in this case her health may still be very good, but constantly worse and worse. healthy/unhealthy isn't a binary thing, there is a gray scale between the two. You can gradually push yourself towards the wrong end of the scale without noticing anything until you reach a critical point. The same is true about a lot of things, one of the most famous is type2 diabetes. Diabetes is not a binary yes/no, it develops gradually but after some time the individual reaches a point where it needs constant treatment to be able to keep an acceptable standard of life.
There are many things that contribute to the tendency of your health change but obesity is obviously a huge negative. It may happen that some other positive things balance it out to a zero-ish change but I highly doubt this.
Yeah but defining obesity and equating obesity to ill health is where everyone messes up.
I'm stating that obesity is a factor that constantly pushes your health towards the wrong end of the scale. Obesity is not necessary in direct correlation with the current state of someone's health, but it affects the tendency of the change of your health in a negative way.
We can define obesity in many weird ways. This is how I define it: If one can grab a considerable amount of fat under her skin then she is obese or she is on the right way to become one in the future. Another definition: If someone is thinking whether she is obese or not then the answer is yes. There are some people (but mostly men have the tendency to develop this) who gain most of their fat exclusively around their guts. I knew some guys who were very quite skinny, but they had a considerably large belly. Some say that this is the most dangerous form of obesity as it has more potential to screw up the chemistry around your essential organs (and I wouldn't be surprised if it would make the work of your organs more difficult because of physical things).
Today someone who can eat basically at any time of the day (and of course lives with this opportunity just for the sake of fun and joy) does not need any overweigh.