The Marines are a much, much smaller organization. The Marines have 203,000 + 40K reserve, the Army has 1,097,050(2008). The Army is actually bigger than the 4 other branches combined (Air Force, Navy, Marines, Coast Guard).
Besides, the Army and Marines have different doctrines. The Marines are overall, a faster response force, but when it gets down and dirty, they need the Army for the heavier firepower that the Marines lack. You can easily tell that a Cobra, while small and easy to deploy, wouldn't hold up to heavy duty work under all conditions as well as an Apache.
Marines get much less funding (They cost about $20k less per marine than the cost of another service's member), because they have to get money from the Navy.
In the end, the Marines can do the hybrid role very well, but aren't necessarily the best at each individual thing which is why the Army's kill count will always surpass the Marines.