In fact, just about the only completely uncontroversial fact about the increase in obesity is that it is entirely due to environmental changes of some sort. There’s disagreement on precisely which environmental changes, and on the likely public health impact, but not on the general principle. The reason is very simple: the genes of this generation’s children came from their parents, with almost no changes. There simply hasn’t been enough time for genetic differences to contribute.This is absolutely correct. Now it is possible that some genes make particular individuals more susceptible to particular environment insults. But then it is unclear what the "cause" of the epidemic is: the susceptibility or the actual exposure. In infectious diseases we are quite clear that some people are immune to some infectious diseases. But if a particular individual becomes sick with a disease, it makes a lot more sense to blame the pathogen and not the genetic vulnerability of the person.
So when we see such a huge change in the prevalence of a chronic disease (like obesity) it makes a lot more sense to ask what is the underlying exposure. In the same sense, I am unimpressed with arguments from things like self-discipline. It is more probable that the current generation is more dissolute than the preceding one, but it seems unlikely that you are uniquely dissolute from a historical perspective.