I can't really respond to an anecdote in any kind of useful way. There are simply too many factors that could effect this. What I can say is that the data does not support what you are claiming. This doesn't mean what you are claiming is incorrect, it means nutritional science is very very difficult and we don't have good evidence that supports your claim.

Right now there is not reason to think obesity happens outside of a caloric surplus, and there is a fair amount of basic science that supports that view. If that view were not true, it would invalidate many things that we think are true about the physical world.

That being said, while calories-in/calories-out is basically inarguable, there are many things going on that can effect calories-out even if calories-in remain the same. Body composition is one thing. I could definitely imagine medications that are catabolic leading to decreased muscle mass and increased fat storage. But I still don't think this would account for a massive change in bodyweight, not one that would make the difference between obese and not obese.