A well documented blog post, "How Video Game Breasts Are Made (And Why They Can Go Wrong)," by Patricia Hernandez explores the history of breast design and programing in video games, with an interesting nod to fan modding of breasts. This is also a subject that has been extensively explored (all the way to second base) by gaming scholars.
Hernandez notes that,
Plenty of people theorize about why games often feature bad breast physics, but there is little hard information about the actual breast-creation process. After looking into it a bit, I found that many amateur developers seemed to genuinely have a problem figuring out how to tackle breast physics in their games. There are a startling number of forum posts and tutorials where people discuss the best ways to achieve good breast physics online. One person even created a four-part Powerpoint presentation titled "The Quest for Boob Jiggle In Unity."