Morality is a moving field. Its scope changes over time. Things which used to be considered an appropriate object of moral approval or disapproval are no longer, and vice versa. It’s difficult to say if the field is becoming larger or smaller. I would guess smaller, but that’s a very uninformed guess. It’s based on my impression that we tend to become more tolerant of each other’s behavior, especially when this behavior is strictly self-regarding and doesn’t involve the risk of harm to others. For centuries, mutually agreed divorce, homosexuality, masturbation, suicide and a large set of other types of behavior were not deemed morally neutral. Many of those could even land you in jail or in hell. Today, in a lot of societies around the world, these things are considered private choices that should not be interfered with and should not even be judged by others. Perhaps some of those behaviors should also be viewed as “rights”.
But even some types of other-regarding behavior that used to be seen as immoral have become acceptable over time. For instance, many types of speech were once considered harm producing – blasphemy, pornography, lèse majesté etc. – but are now believed by many to be less harmful or not harmful at all.
And yet, the reverse movement has also occurred. Slavery used to be morally neutral or even morally required (see Ephesians 6:5) and is now considered one of the worst evils. Cruel and unusual punishment used to be completely acceptable and even enjoyable. The proper treatment of animals, women and indigenous people has also become part of morality, or has shifted place within morality.
So instead of claiming that the field of morality is shrinking, it’s safer to say that it’s moving. Moving where? I guess morality has been moving away from private and self-regarding behavior and towards social behavior. I think this movement is on the whole salutary. Moral progress? To some extent, and certainly not in all domains. On the other hand, the movement of morality can convince some that all morality is just subjective and relative and that everyone can do as they please. If opinions about morality change, then morality is perhaps no more than subjective opinion. That wouldn’t be progress at all, of course, since it would destroy morality completely. Morality can never be completely subjective since it implies a judgment about how others should act and what others should believe.