Why do some women show earlier than others?

Uniqueness. All women/people are created differently, and factors such as muscle tone, the basic "shape" a women is in, body weight and structure, height, etc, will all play a role in when a woman starts to show.
Body habitus. Depending on a woman's weight and height she may show earlier or later than other women. Of course women with twins or triplets start to show much earlier as well.