Dont accept everything on Wiki as being the total truth its to easy to manipulate the information.
In 1931, the Imperial Japanese Army had an overall strength of 198,880 officers and men, organised into 17 divisions. The Manchurian Incident, as it became known in Japan, was the alleged attack on the Japanese-owned railway by Chinese bandits. Action by the military, largely independent of the civilian leadership, led to the invasion of Manchuria in 1931 and later the Second Sino-Japanese War in 1937. As war approached, the Imperial Army's influence with the Emperor waned and the influence of the Imperial Japanese Navy increased.] Nevertheless, by 1938 the Army had been expanded to 34 divisions.
In 1941, the Imperial Japanese Army had 51 divisions and various special-purpose artillery, cavalry, anti-aircraft and armored units with a total of 1,700,000 men. At the beginning of the Second World War most of the Japanese Army was stationed in China, where 27 divisions were stationed. A further 13 divisions were tasked with defending the Mongolian border, due to concerns about a possible attack by the Soviet Union. However, from 1942 soldiers were sent to Hong Kong (23rd Army), the Philippines (14th Army), Thailand (15th Army), Burma (15th Army), Dutch East Indies (16th Army) and Malaya (25th Army). By 1945, there were 5.5 million men in the Imperial Japanese Army.
From 1943, Japanese troops suffered from a shortage of supplies; especially food, medicine, munitions and armaments largely due to submarine interdiction of supplies and losses to Japanese shipping, which was worsened by a long-standing and severe rivalry with the Imperial Japanese Navy. The lack of supplies caused large numbers of fighter aircraft to become unserviceable for lack of spare parts and "as many as two-thirds of Japan's total military deaths resulted from illness or starvation."
Prior to the late 1920s the IJN did not have a separate marine force, instead it used naval landing forces or rikusentai formed from individual ships's crews, who received infantry training as part of their basic training, for special and/or temporary missions.
In the late 1920s, the navy began to form Special Naval Landing Forces as standing regiments (albeit of battalion size). These forces were raised at — and took their names from — the four main naval districts/bases in Japan: Kure, Maizuru, Sasebo, and Yokosuka. These bases all raised more than one SNLF.
Other SNLF were later raised from IJN personnel in China, at Hankow, and Shanghai, for service in Canton and on the Yangtze River. On 7 December 1941 there were 16 SNLF units, this increased to 21 units during the war. The strengths of each SNLF ranged from the prewar peak of 1,200 to a later 650 personnel. There was also a special detachment in the Kwantung area, garrisoning the ports of Dairen and Ryojun.
In 1941, the 1st, 2nd and 3rd Yokosuka SNLF were converted to parachute units. They conducted more combat drops than Japanese Army parachute units during World War II. The SNLF paratroopers were used during the attack on Celebes, to much lauded success by the Imperial government.
The original SNLF personnel were well-trained, high quality troops with good morale and they performed well against unprepared and unorganized opposition across Southeast Asia. However, when faced with determined resistance, such as at the invasion of Timor and the Battle of Milne Bay in 1942, they often experienced heavy casualties due to their unwillingness to surrender. When completely out of ammunition, they would often resort to hand-to-hand fighting with their swords. They were also responsible for the Manila massacre during the Allied invasion of the Philippines in February 1945, were 10,000 SNLF troops under the command of Vice Admiral Sanji Iwabuchi disobeyed orders and stayed behind to fight the Americans.
In a well known last stand in 1943, 2,619 men of the 7th Sasebo SNLF and 2,000 base personnel at the Battle of Tarawa accounted for over 3,000 U.S. Marine Corps casualties.
To this force you have to add the Navy, who had their own Naval Marines, The Japanese Navy was the third largest navy in the world by 1920, behind the Royal Navy and United States Navy. It was supported by the Imperial Japanese Navy Air Service for aircraft and airstrike operation from the fleet. It was the primary opponent of the Allies in the Pacific War.
(Note: the Imperial Japanese Army also raised amphibious units called Sea Landing Brigades. These 3,500-strong brigades were used to assault and then garrison islands.)
As can be seen there was a strong use of combined service personnel for the multitude of tasks that
emerge in warfare.
Another factor that needs to be calculated is the combat loss ratio to manpower replacement ratio throughout the war.