I've wondered that myself, I've actually been there and I didn't think it was THAT big!  So it was a surprise to read that it used to be the largest...  Even though I can't always trust Wikipedia, I don't think I can trust the original source either even though it's Place Bonaventure's official website!  They can at least state in what category it was the world's largest building if it ever held  such title.  Maybe the largest multiple use building?