Biopsies by design are a SMALL sampling of tissues, and unless the suspect tissue is some extremely small potential carcinoma in situ, doctors do not routinely try to take the whole thing. It sounds when you say the word mass, that this was not small, so it would be normal for the doctor to not attempt to take everything. If it is positive in that pathology report, then scans and other diagnostic tests will determine how much more or less extensive it is than what is superficially visible.

Doing too much cutting before full extent of the disease is known is not a good thing, and may subject the patient to the loss of structure/function that when removed, cannot be replaced.

Location also dictates some of this as proximity to neuro vascular bundles make removal of larger areas ill advised. So it seems that the doctor did what was cautious and prudent and not overtly and unnecessarily aggressive.

While you have likely read somewhere that cutting into a malignant area could spread cancerous cells, and that has some validity to it, it is a necessity to remove some tissue to determine what the situation is before attempting to remove things, have a diagnosis, and then after more tests a full staging of the tumor. The risk of spread compared to the gain of knowledge as to what it is, is a trade off that is done everyday of the week by surgeons.