Because B is predicted to scale proportionally to total M 第一个to和第二个to的作用

The first "to" in the sentence is used to indicate the relationship between B and total M. It is used to show that B is expected or predicted to increase or decrease in proportion to the total value of M.

The second "to" in the sentence is used to indicate the unit of measurement for total M. It is used to show that the value of total M is being measured in a particular unit (e.g. dollars, kilograms, etc.).