A muscle mass distinction likely existed for some Marines, but on average most people back then didn't pay much attention to building their bodies in any meaningful way, then like others have said, it's not as if the Pacific War was largely a conflict of bare knuckle boxing.
Warfare, however, is still a contest of being able to run faster than a rifleman or machine gun can target you, being able to overpower an opponent in a close quarters fighting, of being able to effectively fight after a long march in 115 degrees, etc. There's a reason why they put recruits through a gargantuan amount of physical training at boot camp- there's more than aiming a gun in war. To imply that strength does not play a significant part of the overall quality of a soldier is ridiculous.
Any muscle mass superiority of the Americans would have been rendered useless as soon as they found themselves on a pacific island with little food and being plagued by malaria anyway. The muscle mass just dropped off after a few weeks of being there.
Laughable to suggest that there was any such thing as wide spread food shortages being a problem for the allies in the pacific- further inane to say that these nonexistent food shortages, save for whatever might naturally occur in any war in certain unusual situations, actually caused the entire fighting force of the USMC, which would've been more than a little bit physically active, to lose its muscle mass of its fighting troops.
Let us remember that it was the Japanese who resorted to cannibalism at Kokoda and the Aleuts, whose logistics train was still largely pack animals and pack soldiers, and it was the allies who had a gargantuan fleet of 2 1/2 tons and other trucks? Do you really have any idea of the logistics situations of the Pacific campaign? Sure, exhaustion can be a problem for any unit on the line for too long and reduce their combat effectiveness, but the allies rotated their troops out on a regular basis from the beginning of the war, not only did the Japanese not do so, but they COULD NOT do so, as in ever major decisive land battle of the Pacific campaign when the Marines were on the attack the Japanese were outnumbered by a large margin- which also furthered allowed a greater deal of rest to the USMC forces.
Perhaps at first, but a lot of the Japanese troops had been fighting in wars before many of the US Marines had even been born, so I imagine, even with the worst starting training, those years of prior experience would have turned even the most ill-trained Japanese soldier into a force to be reckoned with.
This quote in particular is just plain sad. A lot of the Japanese troops had been fighting before many of the combat marines were even born?
So, an 18 year old marine fighting at Guadacanal, 1942, was born in 1924. Are you seriously suggesting that the IJA mainly consisted of
World War 1 veterans? That the Japanese were using 40 year old troops as mainline combat infantry, and that someone consisted of an advantage? The soldiers bones cracking after they get up from their mid-afternoon nap will alert every enemy soldier within five miles!
You also seem to hold to a fallacy that every single soldier- or even a large percentage of the soldiers in the pacific islands- were veterans of the Chinese war (a conflict which only truly erupted into a full scale war in 1937). Japan was conscripting left and right just as every great power was at that time- so do tell how 'the most ill trained Japanese soldier' will somehow be transformed by his nonexistent experience into 'a force to be reckoned with'.
And let's also examine the fact that throughout the entire war, Japanese casualty rates were horrifically high due to constant illogical counterattacks and a steadfast refusal to retreat or surrender. Call it 'Hollywood' or whatever, it simply cannot change the fact that over 99.7% of the Japanese soldiers on Iwo Jima were KIA. Peleliu, Saipan, Guam, Guaducanal, Papua New Guniea, same story. Japan did not have any vast reserves of veterans- and there disregard for losses left only armies of unexirienced conscripts the longer the war went on, as entire battles such as those mentioned saw every single Japanese soldier die or become a POW- none returning. All the same while there were more and more allied veterans being created by those same battles the Japanese did not survive in.
Do also take the fact that allied aircraft like the Corsair and Hellcat were scoring K/D rations of a gargantuan 12 to 1! 12 to 1! Let that go through your head for a little bit. There were no easily nameable great allied aces throughout WW2, unlike say, the Germans with Erich Hartmann, who scored over 300 kills, because once any western allied ace started getting into the double digits, that ace was restricted to flight instruction to train the recruits, instead of being thrown into battle again and again. This furthermore created an even larger divide in the expirience levels between Japanese and Allies- and not in the Japanese favor as you seem to believe.
I don't really see how that's the case? I'm 100% English, but I can't tell you much about the English Civil War. I'm also 100% European, but don't ask me to explain the Swedish-Novgorodian Wars or the Friso-Hollandic Wars to someone.
Looks like you can add the Pacific war to your list too.