I think that's sufficiently explained by noting bigness makes you more worth suing, and the associated power and high stakes tempt you to do stuff that deserves suing.
Nobody makes scathing documentaries about Burger King, Target or Ford. McDonalds, Wal-Mart and GM are the lightning rods for our hatred of bigness. The smaller guy's legal fees are also way lower in relation to their size than their bigger counterparts. If you have an economist login check this out:
http://www.economist.com/opinion/displaystory.cfm?story_id=E...
