Strong growth at its smaller rivals is a good thing for the world's largest cruise line operator.Two of the three largest cruise line operators have posted quarterly results over the past week. Don't expect to hear from Carnival Corp. (CCL -2.87%) anytime soon. The market leader operates on a different fiscal year that ends in November. Carnival's next earnings call won't take place until late June.It's still important for Carnival shareholders to check on how its rivals are faring. Royal Caribbean's (RCL - ...