The biology behind gaining or losing weight is simple. Simplified, the body is like a house which needs to be kept at a certain temparature all day long, so you have to constantly add fuel to the fire (which is measured in kilo-calories, kcal). The bigger the house is, the more fuel you need to keep it warm (which is why overweight people can eat more than normal people without gaining additional weight), and if you move or work out, you also burn more.
However, your body can only tell you how it responds to food: How filling it is, how it tastes and so on. It has no built-in sense or mechanism to determine how many kcal you already ate. There is often, but not always, a correllation between these: The more bread you eat, the more filled you feel, the more kcal you have is a positive example. But if you eat cake, then you also feel filled (eventually), but a short time later, you are hungry again, even though cake gives you a lot of energy in form of kcal. It is a common misconception to think that just because I feel filled, I had just enough and not too much (or too little).
Furthermore, "healthy" food can still contain surprisingly many kcal. In other words: You can get fat on healthy food as well. That is especially true for fruit or juice, but also for e.g. nuts. If you decide to start your day with a fresh fruit smoothie spiced with Chia seeds, your daily kcal intake will go up dramatically (unless you substitute one meal for the smoothie)
Additionally, our body tends to prefer food which was hard to come by not a long time ago: Sugar and sweets, salt, that kind of thing. The reason is that the body stocks up on things while it can, to depend on it when nourishment is harder to come by.
The combination of all this can make it challenging to keep a normal weight, even when eating mindful. Many people can get it right intuitively, but many others don't.
One more thing:
If someone is overweight and then starts paying attention to what they eat - even with the above in mind - he or she would still not lose weight, because you actively have to ingest less kcal per day than needed by the body, which is something your body normally wants to avoid.
With all this in mind, your observation of people eating mindful, yet being overweight, is not a contradiction in itself, but can have many explanations. The fat could be a relict from the past, before starting to eat mindful, it can be a side effect of a energy-rich diet and so on.
As it is in many cases, in order to change something you have to fully identify the problem - which in this case would mean acknowledging the facts stated above, analyzing the own diet and then changing it.