Twice as Thin?

Here's a math riddle, but an iffy one.  Let me know what you think?

Twice as thin?  The lady down the street claims that on her new diet plan, her waist became twice as thin as before the diet.  If her waist was originally 40 inches, what is her new waist line?

Answer: You  may think and perhaps correctly that her new waist line is 20 inches. (Not bad!)  But the idea of "twice as thin" implies you really think half the the original.  But the predicate is vague and requires an interpretation.  Actually, the "twice as thin" requires a mathematical interpretation as "half the previous."   There is no math here; you have just come to this by habit.  This is a general problem of how vague mathematical-like statements are commonly used.   What would you do if she has said her waist was now 1.5 times thinner?

Now for more practical matters, my blueberry yogurt advertised it that one third fewer calories than their regular type.  What does this mean to the ordinary person?  One third of the calories? Or one third fewer,  i.e. two-thirds of the original?  Does the ordinary person know?