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.
Still, my experience and my circle is limited so I suspect some might have had the opposite experience.
Building features in a large, existing codebase, in the robust tooling context of a big company, is nothing like building services from the ground up.
I think a larger company is probably more likely to have an established mentorship program in place for he engineers rather than a startup.
I also needed to take three months of disability and I received 100% of my pay during those three months (and even kept accruing PTO) because my established company provided me with short term (and long term) disability insurance.
How did this work? From what I've seen, early stage startups always want senior only.
I've been in a junior start-up role twice very early in my career. It worked out for me, but I wouldn't really advise it in an era of Google/MS hiring massive amounts of fresh grads with high comp.
I have my opinions which I will not share for why Hipmunk stagnated but I will say, it had nothing to do with scaling or technical obstacles.