But historically you could take a big island with a couple divisions if even that, or land like 5 divisions and call it the DDay invasion (the beach landing itself, not the whole invasion).
Actually as far as I know D-Day didn't include any marines historically. It was infantry landing supported with airbone assault.
By the way, Allies were triple against Germans (on Utah beach, for instance, there were 4 allies divisions against one bad German one), they had total sea and air superiority, not to mention sabotage, deceptions, German plans to defeat invasion after main beachheads could be localized. Still casualities were comparable.
Edit: How am i supposed to have a real pacific war with that. Seriously.
Historically.
Battle for Iwa Jima had 110K US troops against 20K Japanese. Casualties - 26K for US, about 19K for Japanese.
Battle of Wake. First landing attempt was canceled. Second one had 1500 japanese marines with overhelming naval and air superiority, not to mention sailors, against 450 americans.
Battle of Tarawa. US forces - something about 50K (just 18K were marines) with great support, Jap forces - something about 5K (and about half of this number were conscripted workers). US casualties and Jap casualities again in same book - about 5K both sides.
Battle of Guam, as opposite, was better - 60K US forces against about 19K Japs. Result - about 8K US casualities against full Jap force destroyed.
Okinawa - half a million US troops against about one hundred thousand Japs. Militray casualities - about 70K US casualities against about full japs army (one hundred).
That's how Pacific War was fared. That's why US leaders prefered to use nuclear bombs, not amphibious attack against Japan heartland.
Not "hey, I have magical Marine Division, that just chew out every fortified position with ease". Actually, in WW2 full USMC was
6 divisions in the end of war.
CLARIFICATION: I'm not trying to say current Pacific Theatre is something good. But being not able just to grab everything you want on beachhead with hordes of Marines is an improvement, not opposing way.