I think there are several acceptable ways of saying this, and that the one I've suggested should work. It is quite close to the formulation used in a document on Weighting European Social Survey Data. You could use the literal "aggregated" instead of "combined", I think. Or you could say "the data should be weighted by population size when aggregated/combined".

"3. Population size weights (PWEIGHT):
Population size weights are used when examining data for two or more countries combined. The population size weights are the same for all persons within a country but differ across countries. These weights correct for the fact that most countries taking part in the ESS have different population sizes but similar sample sizes. Without this weight, any figures combining data from two or more countries might be biased, over-representing smaller countries at the expense of larger ones. The population size weight makes an adjustment to ensure that each country is represented in proportion to its population size."
http://www.europeansocialsurvey.org/docs/methodology/ESS_wei...

I think this passage clarifies what they are getting at here: "la agregación de los datos" refers to combining or aggregating data from different regions; the figures must be weighted by the population size of the region to arrive at a reliable national figure.

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Note added at 1 day3 hrs (2015-10-22 08:38:39 GMT) Post-grading
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A pleasure, as always. Thank you!