Not true. I learned more in my first month at a "big company" than in the two years I spent in the startup world. I started out working for startups, and while it was fun at first, it was a big mistake. My development skills ended up stunted and patchy from churning out MVP after MVP. I never learned how to maintain or scale an application, since most of the time I was just pounding code as fast as I could, hoping to score more funding. Since most of my coworkers were also young and inexperienced, I learned little of value and picked up a lot of bad habits. Maybe I just wasn't enough of a "smart young person" to pick ideal companies, but really, who is?

That's just my personal experience, but my point is that startups don't necessarily equal prime education. If you really want to learn and grow, find a team of disciplined people who understand and enjoy their work. Maybe it's a startup, maybe it's MSFT -- the size of a company doesn't necessarily dictate its viability as a learning experience.