Not to worry you but I think it sounds like she may already be on way to having an eating disorder - which doesn't have to be anorexia or bulimia. It can just be obsessive behavior, or anxiety about food or body image.. When it becomes full fledge (where they act it out) it's just an extreme version that can be diagnosable. So I think you're right to have some concern.
It's good you've recognized it now.
All talk about size and food should stop. It's unfortunate she spends her time with friends who obsess about food, diet and physiques. She is witnessing their destructive and self sabotaging habits. If you can gently encourage new friendships and activities that are more supportive and empowering, I would without being obvious.
My best friend had eating disorders and I got pulled into that world and thankfully got help. I was like your daughter - very small and hit puberty late. It can feel like you suddenly become 'chubby' when you finally get curves. Talking about diet and food choices - may not be helpful if she's already feeling anxious. She may already feel she's responsible for the changes taking place (instead of it just being puberty). When it happens later, sometimes you don't clue in that it's just your body changing. If she feels she can reverse the changes, like her friends are doing, she may diet.
I don't talk about food, physiques, etc. here. Our scale comes out if we need to know weight for forms, otherwise, it's not in sight. No one talks about carbs, calories, etc. It's just not a topic of conversation.
Sounds like a lot is going on in her world and her friends sound consumed with their disorders unfortunately (my friend was and they can't possibly be the best of pals at this time. They are going through a lot themselves.). Be supportive and encouraging. Just let her know she's great as is - 'enough' as themselves as the saying goes - and I'd get off the topic all together. :) If she shows signs of anxiety or needing to talk, sometimes therapists for teens are helpful.
My niece went to one at this age, just to talk and it was helpful. Her girlfriends were all gong through stuff and she felt pressured.
Good luck