
After reading this post,I can only agree with above; A phantom can get as bad as the bitch actually having contractions, as if to give birth. It can include all, or any of, the sympthoms of pregnancy. Apart from the actual puppies, that is ;-)
You can get an increase of the width just behind the ribcage. Some bitches seem to accumulate water, and actually look swollen.
But I would be concerned if what you say is huge, is really that; huge.

Have you any reason to believe the bitch could be in whelp? If you are not absolutely certain that no male dog could have gotten to her during her season, then you'd better have her checked by a vet, as soon as possible. When did she have her season? Any changes in her diet lately? Previous history of phantoms?
If at all uncertain, please take her to a vet.
Karen
Edited to add: If I have misunderstood this post, and you are wanting your girl to be in whelp, then I think I might have reason to congratulate you to an upcoming litter