When you first starting out, there are several advantages to going with a small agency. There's a lot less competition, so you are not competing against so many other models in your own agency.
The bookers in a small agency may work a lot harder for you when you are getting started, because they may not have as many high-profile, big-name models who are bringing in the big bucks.
One of the downsides to being with a smaller agency is that each individual model manager has a lot more responsibility than someone at a larger agency. the manager may be responsible for scouting new models, developing them, organizing their books, sending them to see clients, and following up.
He or she may have everything involved with managing the models, promoting and running the agency. This may take away the time it takes to devote to you.
When you are starting your career with a large agency the advantage is that they may have the manpower to take care of all of your individual needs. You can also grow with a larger agency. Also there will be more then one person looking out for you, in addition to your own personal booker.
One disadvantage of a large agency is that even though there are a lot more bookers woh specialize in what you do, there are also a lot more models. As a result you end up competing not only with models from other agencies, but also with you own agency models.Also if your career is not progressing in the way that you r agency had hoped you could get lost in the shuffle because they have so many models to think about. They may not take the time to concentrate on your weak areas and help yoiu improve.