This is a beautiful cup and now I'm thinking of having a cup of tea. Nice dof. As to why you can sometimes enlarge and why not, it may have to do with how big the picture is to begin with. I shoot most things large and then if I crop I can still go bigger but I've shot it small to begin with and I crop it stays the same size. I don't know if that made sense or not.
Are you shooting in jpeg or raw? That might make a difference. I started shooting in raw format back in February and I haven't looked back since then. So much more detail in each shot.
@catsmeowb thank you Camille! I think you will enjoy playing with Elements although i still find it intimidating!