Why do the obv designs have more open areas than the reverse designs on most of the US coins?

fivecents

With most US coins the obverse carries most of the weight when grading the coin, because the reverse design doesn't have as many open fields and open design areas of the obverse. Other than the commemoratives are there any US coins that the reverse carries most of the grading weight due to more open areas on the reverse than the obverse?

bushmaster8

It is a Bush administration cospiracy.