Although most big companies have adapted the CMMI-model, the CMMI can very good be used by small enterprises which insist to have lean and mean processes.
This is very feasible, since the CMMI-model is a set of best practices, and gives no strict rules for its implementation, because this depends on the needs of each specific organization.
E.g. A best practice is that a project must define a Work BreakDown structure. A very large project needs more activities and more details to guarantee good integration, while for a very small, one-room project, the team members know each other very well and so need less detail in their activity description.
It is also possible to combine e.g. CMMI and the well structured SCRUM method. This combination will lead to a different life cycle.
A good book on this subject: