The bulk of the troops in the Pacific were actually Army, not USMC. By my count 22 Army divisions saw combat in the Pacific: Americal, Philippine, 6th, 7th, 24th, 25th, 27th, 31st, 32nd, 33rd, 37th, 38th, 40th, 41st, 43rd, 77th, 81st, 93rd, 96th, 98th, 11th Airborne, and 1st Cavalry. Several other Army divisions were allocated to the Pacific but didn’t make it to the theater or at least didn’t see any combat in theater. In contrast the Marines only had six divisions (all of which saw combat).

This rough 3 to 1, army to marines, ratio existed for most of the war. So while a lot of the Army divisions were only arriving in theater in 1944 or 1945, the same was true of the Marine divisions. 5th Marine and 6th Marine only arrived in time to fight on Iwo Jima and Okinawa respectively; although it should be said many of the individual men had fought earlier as members of one of the other Marine divisions. So late formed Army divisions like the 96th saw about as much combat (Leyte and Okinawa) as a late formed Marine division like the 6th Marine (Okinawa). While an early formed army division like the 7th (Attu, Kiska, Kwajalein, Eniwetok, Leyte, Okinawa) saw similar amounts of combat as an early Marine division like the 2nd Marine (Guadalcanal, Tarawa, Saipan, Tinian, Okinawa).

In practice each battle/campaign tended to have their own blend of forces. Iwo Jima was fought by three Marine divisions with no Army ground units, but with heavy support from US Army Air Force Support. Shortly after, Okinawa was invaded by the remaining three Marine divisions along with four Army divisions. In contrast the Philippines campaign (which was still ongoing during Iwo Jima and Okinawa) was fought with 14 army divisions (although not all 14 at the same time) with no Marine participation. So in early to mid 1945 the major fights were either entirely Marines, entirely Army, or a mix. Most smaller island attacks were combined Marine/Army affairs. Most larger fights like New Guinea or the Philippines were largely or entirely Army operations.

It should also be pointed out that the Australians generally had several army divisions engaged at most times serving in New Guinea, the Solomons, and later in Borneo. For the Salamaua–Lae campaign for example MacArthur was leading one American division and four Australian divisions. It’s an under-appreciated fact that the “American” forces relied heavily on these Australian units, particularly in 1942 and 1943. So there were at times as many Australian soldiers operating under US command as there were US Marines.

So while the Marines were a vital force in the Pacific war, they were only a small minority of the troops engaged in the fighting.