This is caused by the increasing size of the uterus as your baby grows. (twinmom.com)
A minimum of 350 kcal / cup is required and more than 400 kcal / cup is preferred if litter size is above average, as it will be difficult to feed a dog enough food in late pregnancy with the expanding size of the uterus and the decreasing space in the abdomen for the stomach to expand with meals. (langleyanimalclinic.ca)
You shouldn't worry about the IUD altering the size of your uterus, either. (businessinsider.com)