Mammary (milk) glands are developing and hormones cause them to swell.
There is increased blood flow and I think some tissue inflammation as well, but I'd check up that before you quote me!
Im not sure.. I'd google it lol. Im 28wks now & my breast have not gotten any bigger. I have started leaking colostrom every now & then though, which is babys first milk until your actual milk comes in after the birth.
Whwn your going through puberty your breasts only partially develop. When you get pregnant they finish developing.
They are swollen because naturally when your pregnant they are getting ready to be filled with milk. They will get even bigger once you are closer to your due date. With my first child I was a B cup and gradually on my due date I was wearing a D cup so it varies. Just think "hormones." LOL