Those actually are not the same proportions when height and weight increase by the same ratio. To take an extreme, think about a 6' tall man at 200 pounds, and a 3' tall child at 100 pounds. The adult may be reasonably proportioned, while the child is severely obese. The correct proportionate weight for Nida Agdal to match Marilyn Monroe's shape would be roughly 138 pounds at 5' 9" tall (1.0615^3 * 115 . . . actually not quite true for technical reasons; 134 pounds is a better match, but in either case, Nina is proportionately significantly slimmer).
These are nothing if not three dimensional calculations.
The variable here is the bust, for which MM was proportionately larger.
If Agdal had that, she would still probably be 7lbs or so underweight compared to MM.
Good catch and ditto.