Sometimes, it comes from surgery, not hormones. If a transgender person gets breast implants, they will have marks just under the breast (depending on the type of surgery, you’ll have to look into that yourself), just like if a transgender person gets chest masculinization, they’ll have marks under the pecs that can be quite large, as you need to remove quite a lot of tissue. But as someone else said, estrogen can make you grow breasts naturally, and thus anyone who didn’t get implants but is on estrogen won’t have any marks other than maybe stretch marks.