Here is my theory.
Around 1995-1996, Michael's lupus was flaring up and he was on medication to keep his symptoms at bay. Lupus medications often make the patient gain weight. When looking at pictures of Michael on the HIStory Tour in 1996, he is clearly heavier than the other tours. Not in an unhealthy way, but they do have that side effect.
Here's a picture of him doing TYWMMF in 1996. Excuse the, uh, gold pants.
Here's a picture of him in an equally form-fitting suit in 1992 during the Dangerous Tour.
By the Royal Brunei concert, his lupus had already come back. Here is a picture of him in the same leotard in 1996:
Clearly gained since 1992, not because he ate too much or worked out less, but because his lupus had come back.
Here is a shot of him during the HIStory Tour in Helsinki, 1997:
Baggy pants. Michael's thinner. His lupus has gone into remission and he no longer needs the medication, and therefore he loses the weight quickly. Why do I connect it to lupus?
As I said above, lupus causes lung issues and throat issues, which affects singing - especially while dancing. When his lupus went away, he lost weight and started to sing better. Helsinki is, in my opinion, the best show vocally in the entire tour.
The same can be applied to the rest of his life. During the 2000's his weight was similar to the 1996 HIStory Tour. He also lip-synced every performance during this time period, and whenever the mic was live, it sounded extremely rough. However, when This Is It rolled around, he was much thinner and his vocals were FANTASTIC.
Michael's talent never went away and he did not lose his vocal talent to age. His health betrayed him.