There's no real consistency in any of it. Sometimes breasts are PG-13 (Titanic) but usually any breasts are an automatic R (most other modern movies, except for those with sufficient "art cred"). In movies from the 1980s or earlier, breasts could be in a PG movie (Airplane, Logan's Run.)

The PG-13 rating was created in response to violence in PG movies (somebody getting their heart ripped out in Temple of Doom), not nudity.