Yes the US Army had a massive presence in the war against Japan. Both in the form of the Army Air Force, and Army combatant and support units. And yes they did indeed outnumber the USMC in number, though we should avoid drawing comparisons of who really had it worse, there was more than enough to go around.

In the end the USMC suffered about 25k dead and another 65k wounded in the war against Japan, while the Army about 42k dead and 145k wounded, so within spitting distance of double. And that lines up thereabouts with the relative size of the organizations. In 1945 the USMC topped out at about 475k members in every billet from support, aviation, aboard USN ships, to combat arms. And in addition to numerous detached or one off units, fielded 6 full divisions to the USA's 22 which ended up in the Pacific at one point or another, many of these we should note arrived towards the end, to help support the landings on larger islands like the Philippines, or in anticipation of the landings on the Home Islands in Fall 1945. But others like the 7th, 25th, and Americal were present essentially from the beginning, the latter 2 seeing fighting in late 1942 with the 1st and 2nd Marine Divisions on Guadalcanal.

The USMC divisions by 1944 were roughly divided into 2 Corps(III and V Amphibious Corps) which took major central Pacific landings in turn, allowing the other to rest and refit. For example III(with the 3rd, 4th, and 5th MARDIV's) was at Iwo Jima in February-March 1945 while the V landed on Okinawa a few weeks later. This structure was only really formalized in Mid 1943 when the need for larger controlling organizations to support the coming USN drive across the Central Pacific arose. The antecedent, 1st Marine Amphibious Corps had been formed in Fall 1942 to oversee, along with the Army's XIV Army Corps depending on who was in charge, the fighting in the Solomons from Guadalcanal, to Bougainville, to New Caledonia.

We can also note that MacArthur's drive through the South Pacific, around Rabaul, along New Guinea, and then into the Philippines was an almost fully US Army and Allied forces affair.

We should also note the level of fluidity that often accompanied command arrangements in Pacific landings, on numerous occasions Marine divisions were assigned to Army Corps, and Army infantry divisions to the Amphibious Corps. This being more a question of which branch the entrusted or favored ground element commander belonged to, or whose area(Nimitz or MacArthur) the operationw as conducted in. It was only into 1945 as the number of units involved grew that you see the ending of that practice. When you only have 2-3 divisions involved between both branches it doesnt make too much sense to complicate things, but when you have 3-4 from each it makes much more sense.