you don't get fat from eating fat or junk food.
you get fat from eating too much calories, that is, when you expend less than you consume. and all the excuses about being fat or skinny because of having slow or fast metabolism is exactly that - excuses. it's just how much you are used to eat. even if you think you eat a lot, you're just not consistent with it.
i realized it myself when i started somewhat approximating the calories i ate. i felt like i ate a lot, for example often it was like 400g of dry pasta a portion, but i was often skipping breakfasts and i didn't eat that much every day, so it didn't add up.
with that being said, it's not an excuse to eat junk food, it is still really unhealthy and can bring you a lot of cardiovascular issues.