In answer to petknu's question, ATPL is just vast!! It is the sheer volume of stuff that makes them harder rather than any complexity of concepts.
For example, meteorology will cover global weather and climatology as well, navigation will cover different map projections and flying whilst taking into account the curvature of the earth, Air Law is more ICAO based etc.
Basically, APTLs come from the premise that you'll be flying something bigger and faster and more complex than a Cessna 152 and you'll be flying longer distances.
Hope that helps.
Cheers
Whirls