To "grow the majority" means to increase the number or influence of a larger group, in order to have more power or control in a particular situation. Full definition
By growing the majority of fresh produce consumed by local and New York City residents, valley farms also play an important role in promoting public health for some 12 million people. (gracelinks.org)
We are more confident than ever that we're going to grow our majority in November.» (politico.com)
«Based on our record of delivering for taxpayers and their families, we fully expect to grow our Majority next year. (nystateofpolitics.com)