3 sizes of pups

My 2nd Blackbear female (RIP Honey) gave birth to 6 pups, 4 male and 2 females. 2 of the males are smaller, the smallest died due to starvation, as I coudlnt get it to nurse. I have been told this is a rare litter, as 1 is a Roborski and one is a mix Roborski hamster, as the rest are normal...