用 a great deal 指人（people）是强调大量很多,比 many more （people） 还多.Great 就是大,巨大的意思,deal是指度量.
A great deal 在标准英语中基本意思是许多,众多的,大量的,是表明不确指的数或量,数额不详 = an unspecified amount,本来只是用来修饰不可数的名词：a great deal of time,a great deal of work,a great deal of effort.近来也被很多人用来指人或书等等可数的名词.
A great many more people 也是很正确的说法,跟 a great deal more 一样指数量很多,可能比美国化广告语的 a great deal more people 标准一些,不过它是没有 a great deal more 的强调度.
A bit 是指数量极少.如果说 But there are a bit more people in our city than in yours,buddy!语法上没有错误,完全对,不过语气不一样,是调侃的语气.
Much 和 many 不一样,只能修饰不可数的名词（包括abstract nouns = 抽象名词）不能指人：There isn't much food left in the cupboard./ Don't give it too much thought!/ His words make much more sense than what you are saying right now.