By allowing herself to be measured, she abdicates her authority. She steps off the pedestal of "Mom" and onto the scale of "Woman." She becomes an object of study.
The act of stretching the yellow tape around her waist, her hips, or her bust becomes a ritual of validation. It is a moment where objective data (the number on the tape) clashes violently with subjective feeling (the insecurity of aging). Locke plays this dissonance perfectly. You can see the character bracing for humiliation, only to be visibly relieved—and confused—when the numbers come back lower or the same as before.
