There's a reason for that as explained above. MJ was lighter back during Thriller/Victory. & History was MJ at his biggest, I think it's more prudent & logical to compare MJ in TII to MJ in the 80s rather than using MJ heaviest period to distort facts.
I agree though that MJ needed to gain more weight but keep in mind the O2 concerts was a residency concert extravaganza, they were not the stressful tours he so loathed. Though MJ insisted on hating tours he did them anyway for almost every album, he was planning an Invincible tour before 911 happened, after the trial happened, he was supposedly considering continuing that afer the O2 concerts. Still, MJ was most at home during performances or concerts, not tours, but on stage bonding & feeling the love of his fans. Resident stage performances cannot be compared to touring stage performances, the stresses are different.