Pipeit,
For an example drill a 2mm hole right through the whole weld thickness at 6 o'clock on the pipe. The second of three radiographic shots is usually with the source at 0 (12 o'clock) so the 2 mm drill hole will be shown as very close to that size on the film.
If you have a maximum acceptance criteria of 3 mm for a rounded indication then your radiograph has passed but you have a rather large leak problem.
With radiography it is all about orientaion of the source relative to the defect, as jarsanb has stated RT can completely miss certain defects.
Regards,
Shane