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Hm, I don't think so.
The only thing I can think of is that Mia hadn't been aware of her ability before. When she went through the door such a shock hit her hijacked body that she realised she could...

Could Mia be stronger there? In New York? But why?
Maybe she felt that she could have her baby soon and it gave her power...
ehh.. just wondering.

So when Susannah (and Mia in Susannah's body) arrived in New York Mia could grow and attach her own legs to Susannah's body.
But why couldn't she do it in the Calla, in Roland's world? It would have...