My wife was told yesterday that she has a large uterus, negative urine pregnancy test. Why is that?

Ask her GYN Dr. Normal uteruses, like people come in different sizes like people. Or did the gyn dr. Try to explain benign uterine fibroids? Drs. Sometimes explain things to patients, who are nervous & then only partly remember what was said.
Likely fibroid tumor. These are benign tumors that occur inside uterus and make it enlarge. Best to ask her doctor about the details.