Xehanort's Heartless didn't have a physical form. Sora did, even though he was merely a heart, as well.
Kairi gave Sora a physical form by cleasing his heart of the Darkness that took over; XH was just a heart rather than ever becoming a Heartless. I think this was in Report 11 or something like that.
You just said almost the same thing i said except, did you not see Xehanort's Heartless' appearance that's called a physical form if you didn't know.