"I genuinely haven't ever seen anyone that size," Ms Hopper said, adding that Topshop still have not contacted either of the pair.
"First of all I think people deserve some kind of response. They should look at the mannequins because they are not the best portrayal of women's bodies."
"It is fine for us because we're relatively body confident but many teenage girls really struggle".
She added: "They [Topshop] should look at getting different sizes of mannequins".
Although the picture was supposed to only attract the attention of their friends, Ms Hopper said they had been "overwhelmed" by the response with the post receiving more than 5,000 retweets - most of which has been positive.
However: "We've already got a lot of abuse online about this," she said, claiming her friend Ms Bibby has been labelled "obese" by some social media users as others claim they are skinny shaming.
"It is absolutely not about that: it's about the mannequin and how there isn't a variety of shapes in the shops."
Topshop did not respond to requests for comment.
- Independent