Your bargaining posture is highly dubious.... but very well.... I will provide you with a new body and new troops to command. (Megatron: And?!) AND NOTHING!!! You belong to me now! (Megatron: I BELONG TO NOBODY!!) Perhaps I misjudged you. Proceed.... on your way to oblivion. (Megatron: AAAAH!! NO, NO!! I accept your terms! I ACCEPT!!) Excellent!
~ Unicron asserting his power over Megatron before reformatting him into Galvatron.

In works of fictions starring many villains, there can be either a main antagonist for each story arc or a Big Bad standing at the top of the hierarchy. In the latter case, however, one (or sometimes more than one) villain even more powerful and influential than the Big Bad himself can be introduced: the aptly-named Bigger Bad, also known as a Greater Scope Villain. Exceptions aside, the Bigger Bad ultimately has a smaller role than the Big Bad in focus.

However, the Bigger Bad is pretty much the main reason for the circumstances in the story that usually wouldn't be (naturally) possible to occur (pollution, artifact of doom, evil ancient group to which the Big Bad used to belong/is acting as a leader, etc). They can also be the main supply of power for the Big Bad, and sometimes the very motivation for said Big Bad to act villainously. Should the story ultimately end up putting more focus on the Bigger Bad, it may over-cede the current antagonist as the Big Bad. This type of villain often serves as an unseen character or is revealed at the very end of the story.

HIGHLY IMPORTANT: a Bigger Bad can logically ONLY appear in stories with a Big Bad. A Big Bad CANNOT be a Bigger Bad at the same time, unless there is a spin-off series in which one of his servants serves as the Big Bad.