At the great risk of being considered a smug teeny-tiny back in the day, I was 6st12lb when I started work at 16. I was 5'4" tall. I was skinny, always had been and attracted a lot of comments on how skinny I was from being a small child. My entire family is like that though - skinny. We ate healthily, plentifully and together as a family. My mother was a tad coddling and would make us something different from the rest when she knew we did not like, say, pork. None of our friends who were invited to eat with us ever mentioned us having small portions (quite the reverse, really) and would tell others about how great the food was at ours.
People at work, who obviously did not know me, thought I was being starved at home and would question what I'd had for dinner, breakfast etc. whilst commenting on how skinny I was. I didn't think I was skinny, I was just 'me'. I'd wolf as much as my brothers (all older) and doing heavy physical jobs. I'd happily scoff four filled rolls, a couple of biscuits and a can of juice for lunch.
Looking back, people may have thought I was anorexic or such. I wasn't, I was just me.
Fast forward and prior to getting married, I went on the pill <hormones> and my weight 'shot up' to 8st1lb. It hovered around the 8st mark except when under stress (bereavement, divorce, family illness etc.) when I would genuinely forget to eat, never feeling hunger pangs etc. When that happened my weight would fall back as low as 7st without me even noticing, other than clothes were loose and people expressing concern for me.
Apart from times of stress, my weight hovered around 8st for 20 years until different hormones kicked in - peri at 39. On piled a stone or so with zero lifestyle changes on my part.
Further hormones kicked in a few years later - full menopause at 44 - when my body decided from then on to just accumulate every calorie I consumed as fat.
Since then, I've been repeatedly tested for thyroid issues and/or diabetes - always negative.
I believe anyone who says they are just naturally skinny/slim or unable to lose weight no matter how concerted their effort to do so is. I've been both.
Like other posters on this thread, my laugh is of a tinkly nature when those teeny tinies bang on about obesity always being a choice.