Was size 8, now a 6.. but that is just due to a very stressful year. I have always been a 8 though all through adulthood (I'm 46 now).
The thing that I have noticed, or that people have commented on:
I eat slowly. I am always the last to finish my meal. I just cannot shove it all in in huge amounts like the rest of my family 
I really only eat when I am hungry, I don't do a set 3 meals a day thing. If I am hungry in the morning, I will eat breakfast, if not I don't. Same with other meals.
I also rarely finish every single thing on my plate. When I am full, I stop. Used to drive exh mad as I would leave two chips or 3 peas etc.
I do try to get to pilates once or twice a week, it doesny make a difference to weight really, but for toning up. I am not a gym freak by any means, but if I haven't been to class for a while, I can feel it.
BUT.... and I think this is the most important part. I have a very small frame and bone structure. My dd, for eg, who is 14 already wears the same or sometimes bigger size in clothes to me. Not because she is overweight by any stretch, she is built like her dad, muscular and heavier boned.
I believe that as long as you eat well, exercise moderately, don't drink alcohol in excess etc, then you will be the size that you are meant to be. Not everyone is meant to be a size 8. Of course there are exceptions here, stress, health conditions, medications etc.