Is it always necessary to have a c-section if my baby is breech?

No. There is a procedure called an external cephalic version. An experienced physician uses his or her hands to gently move the baby into the correct position. A few years ago a medical study showed than vaginal breech delivery was more dangerous than a c section. Vaginal breech deliveries are not done often anymore. Ask if you are a candidate for external version.
Pelvis size/baby siz. The head is the largest body part that must be delivered & depending on the shape of your pelvis & the size of the baby during labor, a c-section may be the reasonable choice. If premature or small, vaginal birth would be less risky.However, when the margin is thin, you never want to have the bottom half delivered & the head stuck in the canal.