The girl is more than twice your size! How does he expect you to not be angry about this?

Time to do a family-bonding shopping spree. You find the place where you bought the clothes, she tries them on, and he pays for them.

Yeah, she's doing that deliberately. If you were a size or two smaller than her then it could just be wanting to wear your stuff but this? Padlock your wardrobe or just dump the bf. He's clearly not going to even attempt to Parent this situation. He's also clearly not backing you as his chosen partner with the kid. Is the relationship worth him letting the kid treat you like a 'wicked stepmother'? You need to have a sit down and a long think about things.

As a plus-size woman, I'm envious of your size 6 figure and the fact that you undoubtedly have no problem finding clothes pretty much anywhere without having to go to special stores. But I am not condoning what this girl is doing. My God, how would she even BREATHE in clothes that much smaller than her? And if your boyfriend is going to condone this, obviously he favors her over you. He won't discipline his daughter, so this is going to be a common issue. Anything she does will be okay with him, and you're the one that will have to suffer. It may be time to re-evaluate the relationship.