You would have been quite surprised had you been around me before I turned 45. I could eat 4 sandwiches for lunch, or perhaps a who box of Kraft Mac and Cheese. I used to eat before I went to dinner when I was dating my DH because I thought he would have fainted had he seen my appetite.
Now at 62 things have changed, however I still consume more food than any of my children and they all work harder than me (I do not work at all) at keeping weight down. I am not a health professional, however I wonder where you get your information because while calorie intake plays a big part in weight management, there are a whole host of factors that are dancing around in that equation as well.
I also need to say that while you say you do not want to offend, you can take it to the bank that your blanket statements regarding weight, etc are offensive. It is like prefacing an offensive statement with "With all due respect." We all know that the rest of that little comment will not be respectful.
I hope your family members who had that bypass do not regain their weight, because with that surgery weight gain is not related to just calorie intake. Their body's digestive functions are altered forever with that surgery so your premise that their inability to overeat has everything to do with their weight loss might fly in your face.