Pros
Work roles at companies are often less specialized than at large firms. That means employees get to wear several hats, interact with staff more often and are afforded a 360-degree view of company-wide operations. Plus, in a smaller organization, it's easy to interact with c-suite executives and the decision-makers. Because employees have more visibility, it is often easier to advance in a smaller organization.
Cons
A large company often comes with known workplace culture and reputation. That can be helpful when you apply to your next role — recruiters and hiring managers will be familiar with your current company. With smaller companies, you may find yourself spending time in your interview explaining the company, rather than talking about your accomplishments.