[E.Roy]

Cd factors in a cars cross sectional area, which in this case is the same at the front. The problem arises at the rear. The Hatch has way more area at the back, this ends up acting like a vacuum on the car because the air cannot remain in laminar flow as well on the hatch.
If Tractor trailers came to a point at the back, there wouldnt be much benefit drafting them, but because there is huge area at the back, they create a void of low pressure air behind them. In a similar way, the Mazda hatch is fighting this same fact.

Cross wind affect on a car also has to do with surface area, so its way more likely the Hatch will be more affected in a bad way, since it has more area for cross winds force to act on.