I suspect that the huge weight losses are very obese people who have a load of glycogen stored in their swollen livers. Every gram of glycogen is bound to 3-4 g of water, so it is easy for the morbidly obese to shed huge amounts of weight in the first month or two.
My BMI was 59.5 and I lost 10.5 kg the first month and 9.1 kg the second month without weight loss injections, while still eating more than 3000 calories/day. I just changed what I ate in an attempt to reduce chronic inflammation by improving my gut microbiome, the weight loss was a completely unexpected side effect of eating kimchi, sauerkraut, kombucha and kefir every day (as well as healthy fats and as many different plants as possible). Not denying myself anything, but adding a load of different things that I wouldn't normally eat.
I only have 13 kg left to hit a BMI of 24.9 which is just inside the normal range, and I'm not in a big rush to get there. Having lost 80kg so far there's a lot of mental adjustments to be done.
I'm still in the upper range of overweight BMI but sometimes it feels as if there is nothing left of me. Exactly half of me has vanished since the end of February 2022 when I weighed 160kg.
It's weird having a skeleton again. Who knew that the xiphoid process at the end of the sternum turns to bone as you get older? Freaked me out the first time I felt a knobbly boney thing where no knobbly boney thing had been the last time I was able to palpate it.
It's a bit like watching the lawn gradually reappear after a long snowy winter in Uppsala. All the crap you didn't know was hiding under the snow is revealed, old tennis balls, forgotten toys, an astounding collection of thawing dog turds etc. I haven't been normal weight since my early 30s - I'm 67 now, and the bits being revealed don't look the way I remember them. My mind is in no great rush to see that shrivelled brown grass.