It's Simple "Don't wake the baby." What are you talking about gurl? If you are a theater actor you must project and enunciate your lines so that everyone in the audience can understand you and follow the story. In film you are Mic'd and you are delivering your lines to actors that are close to you as well as a High Powered, pore revealing camera up in ya face. So technically, you could further the story with just a whisper and a twitch of an eye.
Don't wake the baby! If a casting director or director has told you that you are too "big" for film, Try this technique, speak as if there is a sleeping baby next to you, and if you wake her you will die. Also if you speak this way, there is a tendency for your facial reactions to be more subtle. Which is good and reads more realistic.
Film Actor auditioning for Theater? It's the opposite. Speak up. Enunciate! More life! More Energy. I hate when I'm in a theater and I can't hear what the person is saying it drives me batty! I am guilty of not projecting in theater projects after I have done a lot of film. Bigger! We want to feel it. They Shout at me. It's true sometimes I am competing with an orchestra, lights costumes and sometimes audience members who are hard of hearing. LOL.
I also have to say that it's impossible to be %100 percent objective about how you appear to the camera and an audience. The work should always start from the inside. This is why we need a director. Please start speaking from a truthful and honest place. These are just simple tweaks that I have observed in myself and fellow actors who toggle theater and film.