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That is a good question and I think that there is one of two answers.
1. First, lips are very sensitive skin and are more susceptible to harsh weather. So, it is likely that they are evolved to be smaller so they have less exposure to cold during the winter time which is longer and more severe the further north you go..
2. The size of lips maybe a co-evolved trait. What this means is that Evolution caused some other trait in white people to occur, and the same causes for that trait are the ones that are also causing lips to be small. Here's an example of a co-evolved trait: the shape of a dog's ears is of course related to their breed. Breeds of dogs which have pointy ears or more closely related to wolves and can be accordingly more vicious. However dogs with soft floppy ears like brittanies and cocker spaniels are evolved further away from wolves and are more friendly. It turns out the decades that dog breeders who have bred to make friendlier dogs then ended up with dogs with softer rounder floppy ears over the generations of breeding. So soft round floppy ears are a co-evolved trait with friendliness in dogs.