The Big Bad is the most prominent, powerful, and dangerous villain in a story, and is often responsible for most of the conflict that drive the plot. If there is a hierarchy of villains, the Big Bad is at the very top of it, and may be in control of most or all of the heroes' other enemies. Sometimes, the Big Bad can work from behind the scenes. However, as long as the most prominent antagonists of a story work for someone who was present all along, or who is setting their course of action, then this someone is the Big Bad.
In some cases, there may be a Bigger Bad, who is more powerful and/or evil than the Big Bad, but is much less involved in the plot and only appears briefly, maintaining the Big Bad as the most prominent and recurring villain in the story. Although sometimes Bigger Bads can act as the final boss of the story.The Big Bad can also be the Heavy but often times it is his right-hand or enforce to make them more mysterious.
There are also the case of the Big Bad Duumvirate when two villains of equal importance form a team and share the role, and the Big Bad Ensemble when several villains of equal importance further their goals on their own, usually in a competition.They are Also the Overarching Villains for the Myth arc as they appear the most times.
If there is a villain-based franchise about the Big Bad appearing the most then they are the Big Bad and villainous protagonist.
A villain can only truly qualify as a Big Bad if the story has multiple antagonists and is of sufficient "scope". In other words, not all stories have a Big Bad, and villains cannot be labeled Big Bads when they are the only antagonist in the story.
Big Bads can ONLY be also labeled as Bigger Bads when they are involved in spin-offs in which one of their followers/another villain takes the role of Big Bad. Big Bad Duumvirate and Big Bad Ensemble DO NOT count when these villain are not on equal footing in term of threat and prominence; or when one of these villains is the biggest threat for most of the story.