Do the big agencies get big breakdowns for main roles and are young actors who are with boutique agents out of the loop for a large roles?
For feature films, the best agents work off grids and advance notice of projects in formulation. This knowledge is what gives these agents an advantage. Grids are directories of films and projects in pre-production. These same agents also monitor the published breakdowns.
There are great boutique or smaller agents and there are great big agencies. Agents also put deals together for talent who have a fan base. They approach the studios and projects are created starting with the talent and then adding the supplemental roles.
Your question is specific to films that are released for casting without a star attached. When there is a lead role in a feature film for a young person, chances are it is a very demanding role. If it is a studio picture, and the studio has a top director and producer and they are risking millions of dollars on this film. And, risking those dollars on an untried young actor is very hard. The casting director will maximize their time and likely results by trying to see those actors who have handled lead roles in the past.
The casting director is hired by a director because they believe in the casting director's taste and because the casting director is well coached in the director's vision for the feel of actor they need. Some directors will spend months or a year seeing new faces to discover the one who has "it." The same casting directors who hired star names only for one film may be casting for a brand new talent next time.
The casting directors are open to truly masterful talent and they will receive it from whence it comes. They check big agencies, small agencies, trusted acting coaches, and they check their own records of magical and memorable actors.
If there is a young performer who has an amazing look, powerful training, and who is winning jobs acting and blowing away casting directors on small roles, they will make it to the next level and the next. They will be remembered. Other actors around them take notice.
Hope that helps!
Carol Lynn Sher