It’s an enhanced form of her normal giant body.
Here’s my theory:
Shes long been able to create wings and large bodies but they were weaker than what we saw here. In essence it’s the peak of her current skill with her devil fruit. Her Mil fleur and it’s variants creates a thousand flowers of herself, however, demonio fleur doesn’t have a number. So rather than sprouting her flowers a set number of times, she concentrated and sprouted one flower. The flower flower may not just bloom arms, it’s very possible that Robin was able to Bloom the mental image of what she felt, the demon inside her.
If she can bloom mental images that she can see and picture clearly enough, then her fruit has some insane potential.