NHS worker Kara Florish was horrified to be singled out by the group and posted a message on social media after being handed a card.
She described it as "hateful and cowardly". Kara - who said she was "smaller than the national average and not exactly obese" was "not upset" by the card but worried it could affect people struggling with confidence and eating disorders.
Sean Thomas Knox also posted on Twitter that he saw a woman being handed a card reading "you're fat".
He said: "Her stunned, desolate reaction was very real, then tears."
Transport for London said the behaviour would not be tolerated and urged anyone affected to report it.