In an SMB, roles may be filled by 1 or more people – but there more like hats – and a single person may be wearing many, many hats. One-man IT shop? You’re a hat shop. Dynamic duo of IT crime fighting? Hopefully you have a Batcave for your hats.
In large corporations you have a complex organizational structure within the IT department, it might be a hierarchical tree or a wheel of distributed teams serving different operational needs. Within those large structures, roles can be generally defined by a person or group of people and what they are responsible for.
In a medium-sized team, the hats will shift around more often as there are more people to wear them but the organization is not so large that it is unable to adapt to change. Role optimization in a team of 3-10 individuals is a process where the roles are all independently examined, along with the strengths and interests of the team members and the potential future needs of the organization as a whole.