Yes, I know of tons of applications that have a large code base and use Meteor. Go look at places like ClassCraft, WorkPop, MixMax, FantasyHub, etc. Plenty of teams are scaling both the code base and the app servers to handle thousands of concurrent connections. What is stopping you from trying Meteor, really? Your questions are more focused on the culture of building an app, not about the framework itself.
Very few know this, but probably the largest Meteor app in existence is Knotable, http://challengepost.com/software/knotable
Hey! Very true. Give a shout and say hello over at Knotable.com or to me a at knote dot com