GiveWell argues that Japan may have no "room for more funding," a term they use "to refer to whether additional donations would result in more of a given activity":
We think it’s important to note that aid from larger donors is still at very low levels: about $28 million from U.S. agencies (of which over $20 million is through the military) and $13 million from other governments. For contrast, on 1/25/10 (12 days after the Haiti earthquake hit), OCHA reported $740 million in funding with an additional $1.1 billion in pledges ... My feeling is that these major funders would be sending more to Japan than they had to Haiti if they felt Japan had serious room for more funding. This is particularly true of the U.S. government since Japan is a U.S. ally.