As far as Cabell is concerned, there are pros and cons to both. The trick is making the situation you’re in work for you.
A smaller agency can offer great perks with regards to producing good work. With advantages like less overhead and more personal control over a project, one can stretch their creative muscles a bit without having too many opportunities for the work to be stalled, watered down, or completely thrown out.
And while in a small agency, you will have a much harder time riding the coattails of others, this smaller environment can help you to stand out and make more of a name for yourself. While you can certainly do this at a large agency, there is a greater possibility of disappearing into the background, leaving little for your portfolio.
On the other hand, there can be very little wiggle room in a small agency due to hampered budgets. This can impact not just the ability to execute an idea but also the eventual visibility of that execution through media. Small budget means small media spending. Larger offices allow you to work with a larger budget which, in turn, allows you the possibility of working with higher quality resources as well as greater support once those projects are greenlit.
At the end of the day, with large or small agencies, what matters is this; talent, hard work, and luck. Add to that the ability to forecast a bit into the future on possible accounts and opportunities with your current agency, and always be aware of the risks and rewards that may lay before you.