That's cute and also fantastic that she wanted to work out with you.
My niece used to workout with my sister and I when she was nine. One day she was feeling really sad about being the biggest of her friends (she was not overweight at all but is very muscular) and my sister told her that everybody has different genes and therefore everybody will be different and that is ok and that even people with larger bodies can be healthy and she said "Like auntie Scarlett right? She is bigger but she is really healthy and strong and exercises and runs)

She felt better after that. Know she is 15 and has a great body image and confidence.
I think it is really important for children to see the adults in there life being physically active no matter what size they are.