Every square meter of Spain could be farmland and the United States would still have 3.5x as much farmland (that is, Spain's total landarea is less than a third of the United State's cropland).
As it stands, the United States has about an order of magnitude more cropland than Spain[1].
Not sure that's fair if the United States is exporting a larger percentage of it's agriculture (as in, that water is indirectly being consumed by other nations). So we'd need to account for who exports / imports from each nation.
Regardless, the United States has 1.4x as much cropland per capita, which already accounts for a good portion of the 1.8x increase in water usage per capita.
As it stands, the United States has about an order of magnitude more cropland than Spain[1].
[1] http://www.wolframalpha.com/input/?i=farmland+in+united+stat...