I understand your concern, but it will be alright. My daughter is 7 and I started noticing breast growing on her around age 5 1/2. My daughter was really skinny until that age and although she is not fat, she is chunkier( and taller) than most other girls in her grade. She is 4 ft 1, and is wearing sizes 12/14 girls in pants, and size 16 girls in shirts( because of her chest).
I also developed early.I was thicker than most other girls. I was wearing a full size bra ( b cup) by the time I was in the 3rd grade, and before the 3rd grade ended, I had started my period. I was only 8 yrs old! And I was not alone! Right before I started my period a new girl had started at my school in my class, and she had already started her period and was wearing a bra also! Just knowing that I was not the only one made all the difference.
That girl was thicker and a little more developed also. Me and my child are very active, and my daughter has normal eating habits as well. Your daughter may just be experiencing her growth spurts earlier.
I took my daughter to her pediatrition also with concern because her growth happend so rappidly, (she went from a size 7/8 girls to a 10/12 girls in 2 months), and he found absolutely nothing for me to be concerned about. Not even her weight, which went from 64lbs to 79lbs in those same 2 months. As a matter of fact he said she was perfectly healthy, blood pressure wise and all.
I would check your family tree on both sides to see if some of the girls in your family were thicker as well. Genetics plays a huge role. My mother and father were skinny, as well as my older sister( who did not start her period until her teens). All my mother's brothers werer skinny as well, as well as her mother and father. However, my mothers grandmother was a thick woman, and so are some of my aunts on my fathers side. I would also check your diet. Switch to low-fat, or non-fat milk, or try soy milk. Also fruits and vegetables always work. Eat lean meats like chicken, fish turkey,, you can make anything out of ground turkey or chicken. But don't make it a big deal. Because your daughter is going to start to feel like there is something wrong with her, when there is not. Just let her know that her body is still changing and as long as you are eating right and exercising, being a little thick is alright! Start to educate her on puberty now so that when it happens, her period will not scare her. Teach her to love her body no matter what her size, and she will grow up with all the confidence and self-esteem that she deserves to have!