when do domain-adaptive pretraining, seems can not extend the vocabulary？

After use my own corpus to do domain-adaptive pretraining, the `vocab.txt` is the same size with the initialized model(BERT-base). In short, the domain-adaptive pretraining does not extend the vocabulary of the new domain? Therefore same specific
vocabulary of the new domain still not exist in the domain-adaptive pretraining result `vocab.txt`. Is that?