I suppose it would not be an exaggeration to say that tomorrow’s election will be the most watched election ever for the regional parliament of Catalonia.
I certainly do not know anything about Catalonian politics, but indications are that the vote will be close between parties favoring secession and those favoring remaining in Spain (while perhaps advocating a new set of center-region arrangements).
The electoral system is quite proportional. At least according to the Wikipedia page on the 2015 election, the assembly size is 135. There are four districts, which makes for a mean district magnitude of 33.75. With a seat product of 135 * 33.75 = 4556.25, we should expect an effective number of seat-winning parties around 4.07. However, in 2015, the actual value was a good deal lower than that, at 3.60 (or 88% of expectation). This was due to a large leading party, which in that election was Together for Yes, with 62 seats (45.9%). Based on the seat product, we’d expect a largest party of around 35% of seats.
Naturally, with such a high seat product, the differences between percentages of seats and votes should not be large. Together for Yes had 41.3% of the votes (ignoring blank votes); overall deviation from proportionality is 3.78% (Gallagher index). While not especially high, that is a little more disproportional votes-to-seats translation than I would expect. There is a legal threshold of 3%, applied in each district. But with the smallest district having a magnitude of 15, this would not have much effect. Besides, the wasted vote percentage (non-blank votes for parties winning no seats) was only 1.1%.
So where does the disproportionality come from? Some of it is from the use of D’Hondt allocation, but with such a high seat product, even D’Hondt should not swing six seats to the largest party (the difference between a purely proportional result, to the nearest integer, and the actual result, for the largest party).
The small but potentially significant disproportionality of party votes and sats must be due to malapportionment. The largest district has 85 (!) seats. This is, of course, Barcelona. That makes it one of the very largest districts in use anywhere. Obviously smaller than the single districts in Israel (120), the Netherlands, and Slovakia (both 150), and formerly in the Russian Federation and Ukraine (both 450), but one of the largest in any recent time in a districted PR system. (I recall Indonesia used to have an 85-seat district; Brazil has one with 70*.) However, it is plausible that Barcelona’s 85/135 seats is under-representative of the capital district’s share of the population. The source I am using does not have vote totals, let alone populations, by district. Maybe someone reading this has that information.
It is actually pretty striking to see such disproportionality in a system in which the range of magnitudes is 15 to 85! Not that it is highly disproportional, but relative to what we might expect from such high magnitudes and seat product, it is on the distorted side. And, in a close election with a crisis over the territory’s status, the electoral system may prove decisive.
* Some Brazilian states have larger, statewide districts for their assemblies. (It may well be that no district has ever comprised a larger percentage of an assembly’s total seats, except where it is 100%.)