I think it's definitely a combination of everything: genetics, emotion/stress, portion sizes, exercise and what you eat. I'm a size 6 (before you shoot me, FairLady, I no longer have any boobs at all after having 2 DCs - hardly fill and A cup and used to be B-C so there is some justice I guess ... ooh, might be a new thread there ...! ) ...
Anyway, what I'm trying to say is that I think we all have different reasons for the weight that we are. Food-wise, I love love LOVE food but I can't eat very much of it at one sitting - I just feel full quite quickly, and if I have something stressful or emotional going on I find that I am quite simply unable to eat at all. I do eat chocolate, biscuits, butter etc but not in particularly large quantities (one of my best friends is a much larger lady and her idea of a "regular" portion is surprisingly larger than mine). I do exercise almost every day tbh, even if it's just walking the dog. I also run around a lot after my exceedingly active kids (my DS is known as Tigger on Speed by many who know him) ...
I'll gladly analyse my diet over a couple of days if it helps you FairLady but at the end of the day I don't think there's a one sieze fits all answer ... Everything in moderation, I think