People with low BMI aren't more active, they are just less hungry, 'run hotter'
I think the hungry bit is often under-discussed.I lost 75kg over 3 years, then put exactly 75kg back on over the 4 years after that.While losing weight, and keeping it off (for about 6 months), I was hungry all the time, every day. I hoped that my body would eventually "get used" to the new standard, but in the end (not conciously), I gave up -- it was too mentally tiring to be hungry whenever I was awake.I'm sure lots of it was a combination of physical and psycological -- I could eat lots of raw vegetables and not want to eat another carrot -- that wouldn't stop me eating something fatty/sugary.