There is a mentally and spiritually healthy way to approach obesity and its relationship with shidduchim.
Men and women have certain issues that are viscerally tied to their feelings of self worth and image. If you knock a man’s torah learning or financial achievements, it is a dagger through the chest…with women, it is their appearance, their housekeeping, and child rearing which are the most vulnerable aspects of their self esteem.
Women who are overweight often look, ironically, to food as a comfort for their lack of self worth. Giving plans for dieting can often backfire…there is an appropriate time and place for such coaching, but it is extremely delicate. Reminding a woman that she will have a hard time getting married usually will only amplify those feelings of self loathing.