Maybe I'm over-simplifying this but...
Isn't this exactly what you would expect? If you drop say 25% of your body weight, would it not follow that your BMR would go down around 25% as well? You are now a smaller person, likely more healthy too, so your body just needs less energy than it did before.
Again, I'm just making up numbers and I'm sure the correlation is not linear, but I think it makes sense that your BMR would go down as you lose weight.
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