Boring question #78; sonics aka silencers are they that quiet really or does size matter?

Being a big fan of the spy thriller sonics (aka silencers) they re used often to muffle the shot of the weapon. In film, they are quite quiet, but I wonder how quiet they are in the real world? I noticed when a sonic on an automatic pistol it is almost as long as the pistol, which makes me think if it was shorter would it be louder? If you had a shorter but thicker one, would it be the same as a longer slender one? It is something to ponder when you have time to spare.