In the class action space, the top five firms control a large portion of available "market share" by settlement and trial verdict value. This is because lead counsel receives the greatest control over and share of winnings in a litigation, and lead counsel is almost always assigned to the firms with the best clients and the longest track-record of winning...it's an auto-catalytic, self-fulfilling entrenchment mechanism for the biggest and best plaintiff shops. Unless and until one of them fucks up hard, there's little to suggest any of them will lose their dominant positions.
Most of the money is in securities suits, though medical devices, pharmaceuticals, and some product defect cases also make good class action grist.
Having written that, there's plenty of niche plaintiff shops who have incredibly profitable, if narrow, lines of business. You've never heard of Langdon & Emison, a small class action firm in a tiny rural Missouri town, but they're responsible for multiple large settlements against car manufacturers and suppliers, and their partners' compensation would make a NYC biglaw partner's blood boil.
Here's Cornerstone's annual securities class action snapshot:
Counsel takes ~25% of a settlement award in most actions, but only take ~10% in the very biggest, such Worldcom, Enron, and Tyco.
2011 was one of the lowest class action settlement totals in the decade because these cases have a significant multi-year lag between incidences of fraud and settlement. I read in Bloomberg that Visa and MasterCard announced a $6.25 billion cash settlement (+$1 billion in other benefits) to resolve the interchange fee class action. So 2012 looks like a much larger haul.