:As a general rule, adult males are MUCH larger than adult females. A large male is over 7' to as large as nine feet, whereas a large female is 6'-6.5'. I'm guessing your red snake is the male?
Of course other factors for growth rate in any young snake are going to come into play, especially how much and how often it's eating, and genetics. Some animals (just like people) are just predetermined to be larger or smaller than what is considered "average" for the species.
It's not going to have anything to do with red vs. black.