I am not an expert, but here is how I think about this phenomenon based on my understanding of CICO and weight loss.
Larger people drop the first 25-50 lbs quickly because bigger bodies require substantially more calories to sustain their size. For instance, a 300 pound man needs to eat around 4000 calories every day in order to maintain this weight. If he starts doing 16:8 IF and eating 2000 calories per day (which is super easy to do if you’re only eating two large meals), then in 11 days he’s reached a caloric deficit of 22,000, which should result in around 6 pounds of weight loss (22,000/3500). In a month he’ll lose 16-20 pounds.
However, as the weight comes off his body doesn’t need as many calories to sustain its size. So, when he hits 250lbs, he now only needs to eat 3000 calories daily. If he keeps eating 2000 calories per day, then after 11 days he’s at a 11,000 calorie deficit and will lose 3 pounds. He’s eating the same 2000 calories as before, but losing half as much weight because his body is smaller.
With smaller people (like myself), it’s really hard to sustain a caloric deficit as you lose weight. I’m 39F and my weight fluctuates between 135-125. I also exercise 4-5x weekly. When I’m at 135 the Total Daily Energy Expenditure (TDEE) calculator says I can eat around 2025 calories daily to maintain this weight. If I cut down to 1500 then I can lose a pound per week. In 5 weeks, I weigh 130 and my TDEE becomes 1975. If I keep eating 1500 I’m now losing less than a pound per week. And so on. It’s also a caloric deficit that I personally find hard to sustain. I’d rather eat about 1800 calories, so if I’m at 135 if will take me 2.5 weeks to lose one pound. However, it’s super easy to negate a calorie deficit that small with one cookie or latte, making it really hard to lose weight when you’re just 5ish pounds heavier than you’re ideal.
Finally, at 5’6” I would be perfectly healthy if I weighed 140 lbs, so no one should take my measurements as an ideal. I prefer to weigh around 127 for aesthetic reasons but it is not always worth the effort, since I also like to eat and drink margaritas. What I’ve noticed doing IF 16:8, however, is that I’m effortlessly staying under 130 lbs lately, whereas in the past it felt like a constant struggle to eat less than 2000 calories per day to lose those last 3 pounds.