"Document review" is something law firms can dump thousands of manhours into with no real appreciable change in outcome. It ends up being more a question of what a company/individual is willing to pay.
The reason big lawfirms stay in business isn't because they generally get much better results, but rather it's because they have the time and manpower to chase down every single rabbit hole to fully understand the law around the cases they are working. So, instead of a firm giving you a "We are 95% sure the law is this way" with a bigger firm you get "We are 99% sure the law is this way."
The other messed up part of the US legal system is the bigger the law firm, the more time they have to chase down discovery rabbit holes looking for wins and (potentially) legally bankrupting their opponents. "Document A references document B written by Foo? Well, lets have a deposition of Foo and a request for Document B". It ends up looking a lot like refactoring spaghetti code in a dynamically typed language.