Greece versus California

Why is Greece such a big problem for the eurozone when the arguably far-worse financial plight of California is not raising similar concerns about the US or the dollar?
The question seems pertinent given the relative insignificance of the Greek economy - it accounts for less than 3 per cent of eurozone GDP (California provides about 13.5 per cent of US GDP). But I am not sure if I have yet heard a satisfactory answer. Suggestions I have heard include:

* Greece has exposed weaknesses in the eurozone’s political and crisis management systems, which imply it would be ill-equipped if ever a bigger member state was to get into as serious difficulties.
* The US economy allows fiscal transfers between states, to help the weakest. But other eurozone countries might well just let Greece fall into an abyss, whatever the consequences.
* Greece is not really such a big problem for the eurozone, but financial markets/commentators have over-reacted - or are biased against the euro, and will seize any opportunity to talk up the prospect of the monetary union breaking-up.
* The eurozone is only 11 years old, so a break-up is easier to envisage than a break-up of the United States
* Eurozone politicians have much greater freedom to act (irresponsibly) because there is no single eurozone government. Hence the potential risks are greater.

Perhaps all the above are valid, but I am still not sure if they offer a full explanation to the Greece vs California conundrum. Maybe I have missed something?