When you first buy a Big Pig in its initial state, there is no way to predetermine which one you may get. Once you have a Big Pig, upon upgrading it, it could grow up and become one of the many different Big Pigs we currently have in the game.
Big Pigs are Variable Contraptions and reward Lottery Style - meaning you never know what you are going to get! The candy heart pig is much like the brown bat - it's more common and will show up more. But the pigs will also be like the fat bats, in that they will be offered in other ways in the future so you can collect the set!