Rawlsians support government income redistribution to the worst-off members of society. If "society" means the whole world, how much redistribution might be involved?
In other words, what fraction of people in the rich countries might have to give most of their income to people in the poorest countries?
Keep in mind that the poorest Americans have clean water, guaranteed food stamps, and free health care, while billions of people around the world lack such guarantees.