OK I didn't want to derail or take over this thread and I assure you I'm not after some free psychoanalysis!
I'm on my way out but wanted to answer a few things quickly before I go:
Uhtred I never said, meant, or tried to imply that all slim people have to work hard to be that way.
If you look back through my posts, you'll see in fact that I have said the exact opposite, over and over again. For example, I mentioned my best friend at school who was like you and Lurked - no real interest in eating - I expressed envy of this (in a hopefully lighthearted way).
I am NOT trying to perpetuate the idea that no one is naturally thin and that all thin people are exercising loads of willpower to be that way, all the time. Lots of us are, lots of others are naturally that way.
Also - to Uhtred and Tara - ffs I wish everyone would stop getting hung up on the specific food I mention people eating in the gym! What I mean is - and I thought this was perfectly clear, although I was typing on my phone while walking through the rain so perhaps not - that it annoys me when people who ARE overweight AND unhappy about it, go to the gym, exxpend minimal effort, and then immediately eat more calories than whatever they've expended, AND THEN complain that 'even though they go to the gym they don't lose weight'
Is that any clearer??!
I am talking about people i KNOW, people I have discussions like this with all the time. Not you, any of you personally, not an abstract person I've made up, but real actual individuals who express frustration, anger, and envy.
Lurked thank you so much. The only thing I have found really upsetting on this thread is the people (not many) saying that having, or having had, an eating disorder, means I should be BANNED from posting on this thread.
MN and every other similar forum is full of people who have some sort of specific interest in the subject being discussed, very often from personal experience. Imagine if we said no one who'd experienced domestic violence should comment on threads on that subject, as they're obviously fucked up or disturbed about it.
SleepwhenIdie I have too much to say in response to your posts, thank you for taking the time to write them
I will say briefly that I was about that weight in between having my kids and although I found it incredibly easy to maintain my weight (around BMI 22) without having to think about it, I didn't necessarily like being that size. And going to the gym has made me more interested in developing my muscles, my strength, my fitness and a different sort of body type, not the sort of comfy/mumsy body I (feel like I) have at 10.5 stone. I know other people think I look fine at that size. I don't though 
more later if anyone is still interested!