I'm not sure, I haven't managed to write an app that large yet. I'm also not sure how you can outgrow Django, since you can always split off parts of your codebase, or not use the parts of Django you don't need.

I've recently written a flask application too, and I found myself having to reinvent the wheel at every step, and I cursed a lot. Django's ecosystem, both in libraries and in sheer community documentation, is hard to replicate.