I have also always been big. I weighed nine pounds at birth, was always taller and sturdier than my classmates at elementary school, never been smaller than a US 12 since the age of 16. I was the biggest by a long mile on the softball team, though I did the same workouts as the other girls. And I walk everywhere, all the time (never had a car, in high school I walked 4 miles round trip to school every day, the same for a job I had a few years ago). Once in college I went on a diet/exercise program. I did at least an hour of cardio and weights at least 4 days a week, and monitored my calorie and fat intake very carefully. I was very disciplined and didn't cheat once. After 6 months, I had lost 6 pounds. I gave up. My body simply doesn't want to be thin, it never has been thin, and I really don't think it can be thin. I am OK with this. I have to be, or I would spend my life hating myself and trying to be something I'm not, and this I will not do.
One thing I heard a few years ago, and apologies for not having a citation for this, is that people who are inclined to be overweight have stronger cravings and are actually hungrier, more often, than people inclined to be thin. So it is not just a question of us putting in as much effort as those others, we actually have to put in condsiderably more. And for many of us, the meager results of those efforts just don't make the struggle worth it.