Covariant return types generate an extra method in bytecode. This compounds in each subclass further specializing the type.

CovariantReturnTypes.java

interface Thing {
  Object thing();
}

class CharSequenceThing implements Thing {
  @Override public CharSequence thing() {
    return "CharSequence!";
  }
}

class StringThing extends CharSequenceThing {
  @Override public String thing() {
    return "String!";
  }
}

Output.txt

$ javap -p Thing CharSequenceThing StringThing

Compiled from "CovariantReturnTypes.java"
interface Thing {
  public abstract java.lang.Object thing();
}

Compiled from "CovariantReturnTypes.java"
class CharSequenceThing implements Thing {
  CharSequenceThing();
  public java.lang.CharSequence thing();
  public java.lang.Object thing();
}

Compiled from "CovariantReturnTypes.java"
class StringThing extends CharSequenceThing {
  StringThing();
  public java.lang.String thing();
  public java.lang.CharSequence thing();
  public java.lang.Object thing();
}