Paradoxical effects are rare and tend to happen only with psychoactive medications. I see very little evidence for paradoxical effects playing a major role, but obviously it can't be ruled out entirely. I don't find paradoxical effects a convincing explanation for anorexia because of how people develop anorexia. You can become anorexic merely by changing your social circle or even your desired social circle, so why would a paradoxical effect occur in that situation when presumably the environment hasn't changed in terms of chemical consumption.
> Why has the percentage of the population that is obese increasing?
Most likely explanation is that they are eating foods that aren't satiating and that are high calorie, foods that weren't common in the past. Like I said earlier, people who are eating simple foods like the ones I listed don't get fat. Genetics seems to play a role in that some people can eat junk and feel full but most can't.