I told my niece that I was concerned about her weight.
Are you kidding? How did your niece react when you told her you were concerned?
She didn't really respond to me or say anything about it.
I'm sure your niece heard you comments and is thinking about what you said. She may be sensitive about her weight and it's an uncomfortable subject for her. It's appropriate for you to express your concern over her health.
I'm going to blame the parents for her weight.
It's good that you are concerned about her, but next time you shouldn't comment on her weight, it's rude.