But a lot of larger companies have great programs for junior developers that ensure they learn a lot.
In my experience and stories I've heard it seems that you can have an amazing time as a junior developer at a startup. If it's a good one. It it isn't, you'll be absolutely miserable and have no support. Similarly, you can be at a large company with a fantastic program for junior developers and learn a lot. And you can also work at an awful large company where you'll learn nothing. "Startup vs large company" feels like a false choice in many ways.