Why do small dogs live longer?

i think if you look back to before we started interfering with dogs by breeding etc. that dogs probably had a long life, but due to our 'making' breeds such as German Shepherds which have hip joint problems due to breeding, and also King Charles Spaniels which can have heart defects. i think dogs would be ok if we hadn't interfered with them. so overall i agree to an extent, but also think humans have had a lot to answer for regarding dogs. i'm saying this as a German Shepherd owner, who has always been told that my dog will get hip dysplasia when he's about 10 year old or younger. i hope this doesn't happen but it's what has happened due to interfering with breeding.