Well, almost everyone has some physical features they like more. It doesn't necessarily mean that black-hair-girl likes her just for being chubby, it means that her chubbiness has caught black-haired's eye, attracted and made interested in the pink one. But whether that interest/attraction evolves into something more is up to personal interactions.
She didn't trick her, as they probably weren't even acquainted then. She just talked about her preferences(what she thinks should be popular). It was pink haired girl who eavesdropped and misunderstood.
Later, after pink girl became chubby, she noticed her(i forgot if they showed how it happened) and became attracted. That's all, I think she likes her as a person now.
Ain't it kinda bad that the blk haired girl only likes the pink hair girl because she's chubby and does it for her own pleasure?