In principle the work is the same, however, in a large company the processes are more structured, and you will have a higher number of senior medics to learn from and interact with. You will be exposed to different aspects of pharmaceutical medicine and promotional prospects are wider, with salary and conditions slightly better too.
In a smaller company, these advantages may be limited, and you will have the chance to be more 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 build more solid foundations in a larger company. Most medics at this early stage choose to start with larger companies. Later in their career, many choose to work for smaller companies.