This is because of size- being a small dog and us being very big in comparison to them. If we lean over them, they tend to get uncomfortable. Always approach the dogs from a safe distance. If the dogs are showing signs, they are not comfortable then give them space
Hand shyness could also be because the breeders haven't handled them well as puppies. They are only raised in their cages and barely handled. They come to your house with a lot of fear already. One of the best ways to get rid of this is by not hugging them and handling them with your hands stretched out.
You can stand at an arm's length and brush them. This takes away the hand shyness. If small dogs are lifted a lot and they are not comfortable with it, this can also lead to hand shyness.