The log facility for sftp-server and internal://sftp-server is set with SftpSysLogFacility. Here, the informational level logs very verbosely. (For example, stat-calls are logged. This can happen hundreds of times when reading a directory listing.) Usually, you want to use a higher level (for example, notice) for logging SFTP.
Nothing is logged by the SFTP-service if the SftpSysLogFacility is not set in the configuration.