Heavy buyers aren't loyal buyers
Are your heavy buyers also your most ‘loyal’ buyers? Probably not.
Of course, it depends on how you measure loyalty. But if loyal = exclusive, you-and-you-alone buying behavior, then the data show that heavy buyers definitely aren't loyal.
People who only buy the category once a year (or whatever the cycle is) are, by definition, the most loyal. One purchase = one brand.
But exclusive loyalty drops to like 50% even when you add just one more purchase.
In fact, the more people buy of the category, the more brands they buy, and the less likely they are to be 100% loyal. Their "brand repertoire" expands, as they say. (“They” being brand science academics.)
People who are really into ketchup, for instance, are picking up some Sir Kensington's, trying the balsamic vinegar version of Heinz, and grabbing Great Value when they're in Walmart.
This isn't to say people don't have preferences. And yeah, some heavy buyers are fanatical about a single brand.
But you should assume that your heavy buyers buy other brands — sometimes even more than they buy yours.