Why do most Mexicans and other hispanics have full lips?

And although a lot of European Spaniards have thin lips I've noticed---like ultra thin actually---the Mexicans and other Latin-Americans who are mixed with Spanish have very full lips usually like the singer Selena, Jennifer Lopez, Jessica Alba who is half,etc. However I'm white with very full lips naturally. But I am just wondering if the full lips trait in hispanics comes from the European mixture or if the full lips were a trait of the Aztecs, Incas or other Indians they are mixed with?