I think about that issue all the time Michaelad but thinking it and believing it are 2 very different things. I too envy people who are happy in their own skin - all I know is that I am least happy when I feel overweight.
Personally, gradually putting on weight meant that my choice of clothing was becoming more and more limited. Thus I always felt frumpy and uncomfortable in the few clothes I had which could just about be stretched to fit. I simply couldn't (can't) afford to go out and buy a complete new wardrobe in larger and larger sizes. I also noticed the effects of being overweight quite accutely ...... finding that walking uphill, even for a short distance made me out of breath, and worse, sweaty, ditto most physical exertion. On top of that I seemed to carry a lot of fluid on my legs - on my shins and feet, which would leave indents if you pressed your finger onto them.
My whole life - I have been a slimmer person for far far longer than I have been overweight (about 6 years). Therefore, the overweight person really didn't feel like me at all ..... and I felt particularly disgusted with myself for letting myself go and for being greedy. Previously, I had had far more opportunity due to my life's circumstances at the time, to go to the gym or simply to walk much more. Then I got a desk bound job which I hated (hence lots of chocolate) AND, could never go to the gym (I was a single mum, so time and money prevented this). Plus, I guess I was getting older and my metabolism slowed down.
I do agree that from a mental health perspective, it's far better to accept yourself (whatever an individual imagines their flaws to be) but literally, and apart from feeling unhappy mentally, I also worried that my weight (which was right on the cusp of overweight/obese on the BMI index) would end up affecting my health sooner or later. I am now about BMI 23 point something I think from memory and apart from feeling much better (though not entirely satisfied yet!) I also have noticeably more energy. We live at the top of a steep hill and I used to dread walking home - it was a real endurance test - now I don't feel it at all.