Manageable isn't just from Amazon's side, it is also an argument from the side of the plaintiffs. e.g. "This class is too diverse. It will be like herding 300 m

Manageable isn't just from Amazon's side, it is also an argument from the side of the plaintiffs. e.g. "This class is too diverse. It will be like herding 300 million cats, which decreases the likelihood of arriving at a suitable settlement. Better to split this into multiple classes with more specific grievances". Everyone benefits from the class being manageable (except maybe the plaintiffs' attorneys, who just want the biggest class they can possibly find).