1. What are similar in a small company and a big one?
Team work, cooperation and coordination, communication, decision-making, challenges, etc. (Tie in that you are good at these so that you can blend in the new environment.)
2. What are different? And how you can improve?
E.g. big companies are more structured and systemic while sometimes small companies can be spontaneous. You will work hard to communicate with colleagues, etc. (No harm to say such because that means you will work hard to blend in.). Or say big companies have probably have more suggestions from different parties, so you will work hard to seek info, retrive info, evaluate info, and use info effectively.
Good luck.