In principle the work is the same, however, in a large company the processes are more structured, and you will have more senior medics to learn from and interact with. You will be exposed to different parts of pharmaceutical medicine and promotional prospects are wider, with salary and conditions slightly better too.
In a small company, these advantages may be limited, and you will have to be quite independent and innovative. If you are a new entrant you will need to ensure that you have access to senior expertise and guidance. You may get larger roles and responsibilities early on.
So, different sizes and styles of companies appeal to different personalities. As a new entry into the industry, you will likely learn and get more solid foundations in a large company, and most medics in this stage of their career choose to start with larger companies. Later in their career, many choose to work for smaller companies and be “big fish in a small pond”.