Definitely not. I am sure Cantonese isn’t that big in China (everything is bigger in China), so I consulted Wiki for a quick look.
According to English Wikipedia citing a 2004 study:
- Mandarin: 798.6 million (66.2%)
- Min (“Fujianese”): 75 million (6.2%) — incl. Hainan, Hokkien, Teochew
- Wu (“Jiangnanese”): 73.8 million (6.1%)
- Jin (“Shanxinese”): 63 million (5.2%) — prev. incl. in Mandarin
- Yue (“Cantonese”): 58.8 million (4.9%) — incl. Taishan (Toisan)
- Gan (“Jiangxinese”): 48 million (4.0%)
- Hakka: 42.2 million (3.5%)
- Xiang (“Hunanese”): 36.4 million (3.0%)
- Pinghua (“Guangxinese”): 7.8 million (0.6%) — prev. incl. in Yue
- Huizhou (“Anhuinese”): 3.3 million (0.3%) — previ. incl. in Wu
Chinese Wikipedia cites a 2013 study:
- 官話: 84.78千萬 (70.9%) — Mandarin: 847.8 million
- 吳語: 7.72千萬 (6.5%) — Wu: 77.2 million
- 閩語: 7.18千萬 (6.0%) — Min: 71.8 million
- 粵語: 6.22千萬 (5.2%) — Yue: 62.2 million
- 晉語: 4.5千萬 (3.8%) — Jin: 45 million
- 湘語: 3.6千萬 (3.0%) — Xiang: 36 million
- 客語: 3.01千萬 (2.5%) — Hakka: 30.1 million
- 贛語: 2.06千萬 (1.7%) — Gan: 20.6 million
- 徽語: 0.46千萬 (0.4%) — Hui: 4.6 million
German Wikipedia has a table with no source cited:
- Mandarin: 955 million
- Wu: 80 milion
- Min: 75 million
- Yue: 70 million
- Gan: 48 million
- Jin: 48 million
- Hakka: 48 Million
- Xiang: 38 million
But it’s German, and thus trustworthy 😉
At any rate, Cantonese is not second, any way you splice it.
For overseas Chinese, the most common languages are Hokkien, Cantonese, and Mandarin. Although I have no data to support, based on my travels, Cantonese dominates the Anglosphere countries and are overall in more countries around the world, but Hokkien absolutely dominates Southeast Asia and has more speakers than Cantonese.