Cantonese is
not the most used, not even close. Mandarin is far more popular. Cantonese is the modern form of the language of the Yue region (named after the old kingdom in south east China), aka Wu Yue. After Mandarin became the "official" language of China, Cantonese has been limited mostly to the southern provinces, the city of Guangzhou in the Guangdong province in particular (and actually, that's where the name "Cantonese" comes from. "Canton" is a bastardization, and is what westerners used to call the "Guangdong" province.). It's also spoken throughout certain parts of south east Asia, like Vietnam, Thailand and Laos.
Cantonese is not the majority language of China, although there are many Cantonese speaking communities in the U.S. This is because Cantonese was widely spoken in Hong Kong, which was a British owned colony until the 1990s, and much more friendly to the western world than mainland China was. It was relatively easy for people from Hong Kong to move to the U.S., and there was actually a pretty big "exodus" of sorts when Hong Kong was being handed over to the Chinese government, as the people of Hong Kong wanted to live in a Capitalist country rather than a Communist state (although, in hind sight, Hong Kong was allowed to remain fairly independent and capitalistic in the end). There was also a great deal of animosity between the people of Hong Kong and the Chinese government, after the atrocities of the past. The Communist government arrested and killed many people, especially teachers and scientists who disagreed with the government, as apart of their "cultural revolution", and so many people fled to Hong Kong.
There's a graph here that shows over 70% of people in China predominantly used Mandarin:
Chinese language - Wikipedia, the free encyclopedia
Eh, I probably shouldn't have written all that, but I'm bored and need something to talk about. Cheers if anyone bothered to read it, though.