There's a chart here (see UPDATE) showing the percentages of various foreign countries populations who now live in the U.S. The data is from a government source and Pew Hispanic and is from 2007. The figures are probably on the low side.

Leading off, almost one third of Guyana's population lives here (31.8%). Next up are Trinidad & Tobago (21.3%), Jamaica (21.1%), El Salvador, (16.0%), Mexico (10.8%), Cuba (8.6%), the Dominican Republic (8.0%), Haiti (6.3%), Honduras (5.7%), Guatemala (5.4%), and Nicaragua (4.1%). The string of Latin America/Caribbean countries is finally broken by Ireland at 3.4%.

As for numbers of persons, Mexico is of course far in the lead with 11,739,560, followed by the Philippines (1,708,542), India (1,513,953), and China (1,367,772). As a percentage of those countries' populations, the last two are just 0.1%.

UPDATE: swivel . com/data_columns/spreadsheet/10375108 is no longer an active site, so here's a temporary link.