> Maybe they tend to, but when they drew that graph, Italy is definitely an outlier.

Granting that, why bring Italy up them?

> Very proportional systems have large problems of their own too, including all the "horse trading" that goes on to form a coalition.

Horse trading goes on to form a winning coalition in two-party systems, too (the major parties in electoral systems that create a two-competitive-party dynamic are, invariably, coalition parties); the difference is that, in such systems, the disporportionate power of the needed-to-win segment is less than in two-party systems, because its much easier -- because the factions are formal parties that you can negotiate with -- to swap coalition partners and form a new majority coalition if a minor partner wants too much.

(That's also, really -- outside of systems like the US where you've got an FPTP electoral system forcing pre-election coalition building -- a bigger issue in unitary parliamentary systems vs. separation of powers systems -- because having a majority coalition in parliament is a more significant issue in unitary systems whereas in separation of powers systems, ad hoc coalitions on particular issues can function in the legislature without requiring a stable "ruling" coalition in the legislature. So, if you are looking at an FPTP separation of powers system like the US, its a problem that moving to a proportionally-elected but still separation of powers system alleviates rather than making worse.)