I have recently read some where at blogs that Google is going to pannalize the sites having the low loading speed. Keeping that in mind I can say that long lenght URL websites could have the low loading speed, that is why Google gives the priority to the websites having the short lenght URL.
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Not exactly a good criteria for Google, visitors are the ones who wants a shorter domain. Shorter domain are certainly more memorable to someone. Others like to search on the address bar typing only a single word every time they do the search.