Does the uterus return to its original (pre-pregnancy) size after a full term birth?
Good question. No. Uterine involution, the return of the uterus to the nonpregnant state postpartum, does involve a remarkable decrease in size, but it's not as small as it was before pregnancy. Nonpregnant uterine size increases with what we call parity, or the number of times a woman has carried a pregnancy to term. I can't seem to find the original data on this, but it would likely be an early 20th century (or possibly late 19th) pathology series. The phenomenon is referenced in the pathology literature, e.g., in the section on hysterectomy specimens in this article:
Note that the parous uterus (premenopausal adult 75-100 g) is heavier than the nulliparous uterus (premenopausal adult 30-40 g), and weight increases with parity, so that after eight pregnancies a weight of 240 g is normal.
This finding (increased uterine size depends on parity) is confirmed in imaging studies, for example, here.