Belo Horizonte is bigger if you go by urban area population. It's smaller if you go by city proper population. But since the rest of the quiz seems to use urban area, it would be more consistent for Brazilian cities to use that as well. The top 5 Portuguese-speaking cities would be: Sao Paulo, Rio de Janeiro, Luanda, Belo Horizonte, Porto Alegre.
Also Kinshasa, Abidjan, and Luanda aren't majority French speaking, but I can understand why you added them.