This paper provides a helpful breakdown of types of mortalities and how they're associated with BMI. It's interesting to see that the lowest All-cause mortality is at a BMI of 25. A BMI of 30 doesn't increase the hazard ratio that much, but a BMI of ~45 doubles it. The minimum for cancer mortality is notably lower (21) but perhaps surprisingly the minimum communicable disease mortality is at a BMI of 26.
https://www.thelancet.com/journals/landia/article/PIIS2213-8...