In my experience the nipples are hardly visible before the first heat (like in children). After the first heat however they are a lot more prominant. Cleos nipples did not go back to preheat size. They stayed fairly prominant. I relate it back to humans. In humans our boobs/ nipples get larger and never go back to they way they were before we reached sexual maturity. It's the same in dogs I gather. Nothign to worry about
Then when they do get pregnant they look like bullets!!
Hope I was of some help to you!