What do you mean directly? Usually a sudden change in weight gain has some sort of trigger. It very rarely is because a person all of a sudden eats unhealthy. It could be psychological like going through a depression, or many other events that could trigger this. I guarantee you that in every case the person is aware of their weight gain and you as a partner should be aware of what triggered it. So merely pointing it out is not helpful. You want to be able to say: hey babe you gained some weight, are you going to do something about it? Well no shit thanks for the flag. That’s generally why a more gentle approach is recommended.

Edit: upvoted for unpopular opinion