It really depends what team you're on! My little corner of megacorp is fast, pretty lean, and I definitely feel like I am able to make an impact on our product (which is a smaller product in the company). I feel much better here than at some of the mid size companies where I felt there was a low ceiling on growth opportunities.
What I vastly prefer is the 500-ish size. You can realistically at least know of every other developer (and find those you prefer working with and can learn with), there's more resources than just a 30 person company, and you're just not a tiny insignificant cog in the machine at a 100k mega corp.