EDIT: Catching up to this #messy thread. How, Mumei, have you come to the conclusion that Mariah does not have a big voice? Have you so quickly forgotten
this flawlessness?
I have, or rather I have been convinced of it by other peoples' arguments. I haven't forgotten about that performance - I even listen to it out of season! - but I think that demonstrates her projection more than it does the pure size of her voice or her volume. If you listen to her sing without a mic when she belts, she doesn't have a big sound.
I think it is most noticeable that she does not have a big voice when she is singing with brass. If you listen to her performances of If It's Over on Saturday Night Live, or Someday, Make It Happen, Emotions, or If It's Over on MTV Unplugged you should notice that she doesn't sing
over the instruments. She doesn't produce more volume than the instruments. She projects through the din, and you can acoustically hear the instruments surrounding her voice instead of being behind her voice. You can hear an opera singer with a small voice producing even more squillo to do the same thing
here.
By contrast, if you listen to Whitney singing against the horns at about 1:50-something in
this video, her sound is clearly predominant. That's a bigger voice.
Oh, and
this is a good video for why Christina has a smaller voice, with examples of bigger voices that are contrasted with Christina. I think the same contrast can be done with Mariah, though Mariah simply has better technique than Christina.