Mike: As far as I can understand, there is no international standard for caps, or nipple dimensions. That may be shocking, but it seems to be very true.
If the caps will fit on your nipple, but are loose, just pintch the sides before putting the cap on the nipple and that will hold the cap on the nipple. Its been done that way for years. If you think you are having trouble with a loose cap, think about shooting an underhammer gun! As for caps that won't fit on your nipple, either get a new nipple that will work- don't throw the old one out- or trade the caps to someone who has caps that are too loose, but fit your nipple just fine!