Logger shift_age of 0 disables shift_size
shift_age is documented as: Number of old log files to keep, or frequency of rotation (daily, weekly or monthly). Default value is 0.
shift_size is documented: Maximum logfile size in bytes (only applies when shift_age is a number). Defaults to 1048576 (1MB).
Given the above, a shift_age of 0 should prevent the retention of old log files, but maximum size (shift_size) should still be enforced.
So either the behavior or the documentation is wrong.