The argument is that it is too large for the court to handle, not too large for Amazon to handle.

Class actions work by grouping many plaintiffs who have been harmed in the same way and whose damages if they win can be addressed in the same way (e.g., everyone gets a fixed amount of money, or a fixed percentage of what they spent, or something like that).

Then the cases of a small number of representatives of the class can be addressed at the actual trial, and the outcome applied to the whole class.

The larger the proposed class the more likely the variation in the harms to the members and in how to redress those harms, which can mean you need a larger number of representatives of the class at the actual trial.

Get large enough and it becomes difficult for the court to manage. In that case the class needs to be shrunk down to a set of plaintiffs with more uniform harms that can be addressed more uniformly. The people removed from the class might form another class or classes for separate suits, or sue as individuals, or some mixture of those options.

Nothing a small R5 cluster can’t handle