Well there is a good reason for that. You see, Slime girls are Slimes that have retched their full potentiol. And since Slimes love anything that resembles them in size, shape and feel they tend to hang around with girls that have good sized breasts. Since that's the case that is the type of humanoid they spend the most time around, so that's the shape they take when they can. Well that and I really like breasts.