The GIMP documentation states that: "XCF is GIMP's native format, and is useful because it stores everything about an image (well, almost everything; it does not store “undo” information)."

Well I think that it should also save the undo history. I think that it is a very important feature. That way one could always go back to a previous state and change something that was not the most correct enhancement of that image.

The user could define the size of file or the number of undo steps stored in the 
file.