On first glance, the ratio of Japanese to American losses during the island hopping campaign do seem odd. Despite regularly being on the defensive, as you note, the Japanese regularly suffered heavier casualties than the Americans, with the only notable exception being Iwo Jima. As anyone familiar with warfare can tell you, holding a defensive position is generally a significant force multiplier, and a defender is generally expected to inflict greater casualties on the attacker. So what was happening in the Pacific in World War II?

There are several notable reasons for this apparent discrepancy. However, first let's provide some important context for the operational environment in which these operations were carried out. By the time the United States began its "island hopping" campaign in late 1943, the Imperial Japanese Navy had been forced to withdraw inward in an attempt to rebuild its strength after the grueling attrition of the Solomons Campaign. The Japanese had been bled white of ships and men during the Solomons campaign, and with the Allies advancing, the IJN had withdrawn its forces to Truk and anchorages closer to the oil production facilities in the occupied Dutch East Indies. The Japanese fleet would not sortie in strength again until its attempted defense of Saipan that resulted in the Battle of the Philippine Sea.

What this means for American invasion planners, of course, is that they can choose when and where to attack. While the Japanese had fortified as many of their possessions as they could, there were never enough men, troops, and supplies to go around, which meant that some islands were much more heavily defended than others. American planners were thus able to target the more weakly defended islands, and bring overwhelming force to bear against those islands. Just to give an example, Tarawa--the first major opposed landing--was held by around 2,600 IJN base security and landing troops along with another 2,000 or so construction troops (including conscripted Korean laborers). These defenders had no aircraft or naval support and possessed a few dozen artillery pieces and coastal defense guns. The American fleet which attacked Tarawa brought 35,000 landing troops and a support fleet of nearly 40 warships--3 of which were battleships, each of which likely outgunned the entire Japanese artillery park on Tarawa by herself. While Tarawa is remembered as a particularly bloody battle, the fact remains that it was an example of overwhelming force being brought to bear against a relatively weakly held target. To compare, while Tarawa had a total garrison of around 5,000 Japanese in total, the major base at Rabaul was home to nearly 100,000 Japanese service members, while the fleet base at Truk had a garrison of over 30,000 IJA troops, as opposed to the less than 3,000 Naval infantry at Tarawa. These islands, with their much larger and better equipped garrisons, were never attacked, as Allied forces were able to target the relatively weakly held islands.

The same Allied naval superiority that enabled the Americans to choose their battles also impeded the Japanese ability to respond. Large Japanese garrisons on bypassed islands could not be withdrawn, nor could reinforcements be dispatched effectively to support those islands under threat. The increasing efficacy of Allied submarines also played a key role in limiting the ability of the Japanese to send resources and reinforcements forward. In one notable example, an effort to reinforce defense on Saipan with an additional 4 IJA divisions saw the troop convoy severely mauled by American submarines, meaning that instead of receiving 4 combat ready divisions with all their equipment, the defenders of Saipan received only a few thousand men who'd been fished from the water, with almost all their equipment lost. Similarly, the Japanese merchant marine had been straining at the seams in its efforts to support the economy and the military's demands for transportation even before the outbreak of the Pacific War, and wartime losses did not help the situation. Thus, Japanese access to materials like concrete and steel or engineering tools that could be used to improve or construct fortified positions were also limited, forcing defenders to make do with what equipment they had to hand and what resources they could source locally.

Finally, but perhaps most importantly, was the issue of Japanese infantry doctrine for defending an island. Prior to the Pacific War, the Japanese really had no specific doctrine for island defense, and thus instead relied on adaptations of their doctrine for defending a river. This called for a forward defense of the beach itself, followed by aggressive counter attacks in an effort to immediately drive the attacker back into the sea. While these forward defenses and immediate counterattacks did create strong initial resistance to the landings, it also exposed Japanese defenders to the overwhelming firepower of American air and naval artillery attacks. This resulted in the early island hopping battles being relatively quick--if bloody-affairs, as once Japanese defenses were breached at one point, the other outlying positions could be enveloped, while large Japanese counterattacks were targetted by both naval gunfire and air attacks. However, by Iwo Jima, Japanese doctrine had begun to change. Instead of focusing on holding the whole island and defending on the beach, Japanese defenses were moved into the interior, aiming to force the landing forces into a long drawn out fight to clear the Japanese from their interior defenses. This doctrine was reflective of shifting Japanese priorities, as they were now no longer to try and hold the island against a landing, but to inflict as many American casualties as possible, while also delaying the American advance, in order to buy time for further defenses to be set up on the Home Islands.

Hopefully this helps to answer your question, and--as always--please feel free to ask any follow ups.

The short answer: overwhelming firepower.

There isn't any general principle that the defenders of a fortified position will inflict more casualties than they take. Because they are protected by fortifications, the effect of the attacker's fire is reduced. However, if the attacker's firepower is sufficiently greater, the attacker can inflict higher casualties.

How does the attacker bring superior firepower into action? Assuming similar equipment, it is important to outnumber the defenders, at least locally. The defender's forces are spread out, protecting their many fortified positions (or at least those on the front line - fall-back positions in the rear might be initially unoccupied), and in a local action to capture or destroy a particular strongpoint, the attacker wants to outnumber the defenders of that strongpoint. As long as the attacker has sufficient numbers and can move freely enough on the battlefield, this can usually be achieved. Also important is that the attacker can bring their available anti-fortification weapons to bear (even if heavy artillery is unavailable): grenades, flamethrowers, and direct-fire anti-tank guns. Smoke and suppressive fire can be used to cover the advance of soldiers with grenades and flamethrowers, and the strongpoint is then destroyed, with the defenders either retreating or being killed.

The difficulty is that strongpoints are hidden: camouflaged bunkers, camouflaged caves, etc. The attackers will often take multiple casualties as they advance and are fired upon, before they can spot the location of the strongpoint.

If the attacker can bring heavy artillery or airstrikes onto the strongpoint, this can greatly increase their firepower, and can sometimes let them destroy strongpoints without infantry needing to get close. Often, such heavy weapons aren't enough to destroy the strongpoint, but can provide an opportunity for attacking infantry to get close.

Late in the war, flamethrower tanks were available (e.g., they were widely used - and very effective - on Okinawa). As long as the strongpoint didn't have anti-tank guns or heavy artillery, the attacker simply needed to use their infantry to protect the tank against suicide attacks by the defending infantry, and the flamethrower tank could reliably destroy the strongpoint.

The longer range of modern weapons (especially heavy artillery and aircraft) compared to muskets and muzzle-loading artillery (and pre-gunpowder weapons) makes a big difference. It allows the attackers to concentrate a lot of heavy firepower on strongpoints, and forces strongpoints to be well-hidden.

Where the concealment of strongpoints is easy (e.g., in jungle), and terrain limits the ability to use heavy artillery and air attack (e.g., jungle, very hilly terrain) or the attacker has little heavy artillery and/or air support, their job can be much harder. For example, in the US/Australian attacks on the well-fortified Japanese positions at Buna-Gona, their battle casualties were higher than the Japanese battle casualties (about 5,000 vs 4,000). With little heavy support available, the infantry had to do almost all of it themselves, and with the Japanese strongpoints placed for mutual support, it was difficult to obtain the required local superiority in firepower.