The electoral system gives the nationalist parties an exaggerated parliament representation - that has been repeated time and again since Izquierda Unida (IU) lost more than half of their seats – and through South of Watford I have found a chart which can be used to show the absurd effects.
There are two figures for each party, of which the first one shows the actual number of seats in the new parliament, while the second one reveals how many seats the same party would have if the whole of Spain would have been treated as one constituency. With 1,2% of the total vote in Spain, but concentrated to three Basque provinces, PNV gets 6 seats. At the same time, IU receives only 2 seats although they were voted by 3,8% of the electorate. Clearly, this must prove that nationalist parties are always better off than a small party which appeals to voters throughout the state.
In fact it does not. What we see is an effect of an electoral system which systematically gives a higher weight to small provinces although that is where we have the least proportional allocation of parliament seats.
Catalonia, the other nationalist stronghold, serves as a contrast. The chart reveals that CiU would have 11 parliament seats whatever method is being used. The reason for that is that most of the CiU seats come from populous Barcelona, where each individual vote has a weight below the Spanish average and seat allocation is highly proportional.
Take note that not only PNV but also PSOE and PP would lose parliaments seats with a more proportional allocation. Keep this in mind next time you hear that the Spanish election system give an unfair advantage to the nationalists. It is such a nice story that it will be repeated although it is not true.
Remarks: Since the chart used in the article was made, due to late incoming votes from abroad, 1 Barcelona seat has been re-allocated from CiU to PP, i.e. CiU will have 10 seats in the new parliament and PP 154.