D-Day Normandy was June 6th. D-Day Saipan was June 15th.
Saipan was the first of the islands that Japan claimed as "home territory".
http://www.historyplace.com/unitedstates/pacificwar/timeline.htm
The USMC was a small organization at the beginning of the war. The organization did expand and grow but never to the size of the US Army.
Also, as a former Marine myself, I tend to resent the idea that Marines are or were any tougher than US Army as individuals. American fighting men are American fighting men.
Where the USMC and the US Army differed (outside of the obvious TO%E) was that the USMC fighting SOP had to be much more aggressive than the US Army's due to the nasty nature of fighting a war where every battle starts with an amphibious landing on a confined and isolated geographical point where the enemy still has a presence in both air and naval power that can pose a real threat to the logistics tail of the force.
Battle islands had to be fought with such violent force because time was always of the essence and for every day the battle lasted, the fleet was in danger due to having to remain in a position of potential exposure to subsurface, surface or air attack.
Two cases bear this out.
1. Guadalcannal - the US Navy took a beating while supporting the operation.
2. Okinawa - largest US Navy casualty producing battle of the war. The Navy was taking such a horrendous pounding from the kamakazi attacks that it was seriously discussed whether the operation should be halted, the USMC and US Army forces pulled back onto the ships and the fleet sail away to safety.
It's common habit to laugh at the Japanese suicide attacks now days but at that time, there was real fear that the Navy force at Okinawa would be largely destroyed and the US forces on the ground be left to their own ends.