[quonn]

1Password actually stores new data in new files, presumably to make synchronisation work properly. For example, I have approximately 600 such files in my Dropbox. Interestingly, some metadata such as the login URL and the creation date is not encrypted, so it would be possible to build a list of the sites I have stored passwords for.

Formally, if your cipher would weaken in a way that makes practical attacks possible as more data is encrypted, it would be considered broken. Furthermore, it is possible to work around this by rotating keys and just encrypting the keys in a master file. 1Password definitely uses encryption keys that are fully independent from your master password, although I don't know if they periodically use new keys for new data.