The extra science classes don't help with your med school application, but may be helpful in med school when the material will be more familiar if you've already taken some classes covering the material.
Er, isn't psych a SCIENCE? Both a natural and social science, I think.
The comment was taken out of context. I was referring to the additional science classes a biology or chemistry major would have taken. Particular classes you'll take again in med school are anatomy, physiology, and biochemistry - courses biology majors take. While a psychology major will have taken more psychology classes, typically the psychology coursework in medical school is minimal in comparison (both complexity and magnitude).
Psychology is a science, but of a different nature.