I'm not sure of my daughter's exact height and weight at 8, but I know that she felt "big" and was a little heavier than average for her grade. Buying clothes that fit was beginning to be challenging. She was a 14/16 but not as tall as needed for that size. She started wearing an undershirt at 9, and camisoles at 10. I was concerned, too, about early periods.
Now, she'll be 11 in March and she is 5'2" and about 95 lbs. She's still wearing a size 14/16! and she's much thinner than she used to be. She still wears just a camisole, her breasts are true buds now, and pubic hair started at 10. No period yet.
I think it's normal for many girls to go through a bit of a chubby stage, before they grow taller and really start developing. As long as she is physically active, and heats a healthy diet, her normal growth should even things out.
I also strongly recommend the "Care and Keeping of You" books from America Girl. They cover all the things that happen during puberty - hair, skin, moods/emotions, periods (but not sex). My daughter read them intently and asked me questions that opened up good conversations. They are appropriate for 8-9-10 year-olds.