I think yes kids notice, they are not fat shaming, they don't understand the concept. She is just saying what she sees (sorry).
It's well off to say she is rude, she is a child.
It's complex because you now have to explain to her she cannot comment on people's appearance. Which is fine, but To explain to her she cannot comment on her mothers, to say what she thinks, she has to not be honest with you.To her she's just curious and asking her mum an honest question.
It's one of the complexities of life. You cannot tell a fat person they are fat. You cannot tell a thin person they are thin. It needs to go without comment, as it may hurt someone's feelings even if it's true. It's not so easy to explain that to a six year old when in relation to her own mother.
For me, I alwats went with the approach you can't say it to other people, as it may hurt them, but you should always be honest with your mum and dad and ask what you want to know. If it's too distressing for you to hear your daughter mention it, then tell her that includes you.