Mariah Carey's album sales were high because of they were released in the 90's also i don't think you can compare Mariah's album sales to christina's because mariah is alot more popular then she is and has more hit records then she does. I doubt Christina sold 16 million worldwide and i would really like to see a source that states she sold 16 million worldwide because i have never heard that figure EVER before. Also the U.S. is one of the biggest markets in the world i don't think Christina's album really took of or did as well as it did domestically as it did internationally.  

 But if a source is given it would confirm an one who is skeptical.