Maybe she has another hiding along side that pup that is all streached out in there .
NOrmal for them to look big on a x-ray when there is only one they have it all to themselves
I have had sinleton litters they are very special little ones who will want alot of your attention.
Mine have never went over in DD with a sinlgeton and have whelped them fine.
She sure does look like she has enough milk for a few.
Good luck with the whelping