The object safety page says

• All associated functions must either be dispatchable from a trait object or be explicitly non-dispatchable:
  • Dispatchable functions must:
    • [...]
    • Not have a where Self: Sized bound (receiver type of Self (i.e. self) implies this).
  • Explicitly non-dispatchable functions require:
    • Have a where Self: Sized bound (receiver type of Self (i.e. self) implies this).

However, it is not true that a receiver type of Self implies Self: Sized.

// Minimal example; it also can be demonstrated with bounds that
// require `Sized`, like `<G: From<Self>>` say, even without a
// function body
trait Trait {
    // Fails without an explicit `where Self: Sized` bound
    fn method(self) {}
}

Accepting self-receiving methods as object-safe methods was a separate consideration from a Self: Sized bound in the RFC.

This is why all methods that require Self:Sized are exempt from the typical rules. This is also why by-value self methods are permitted, since currently one cannot invoke pass an unsized type by-value (though we consider that a useful future extension).