>Is it still a cult if it gets big enough?

To draw an analogy, if a street gang gets big enough and controls enough territory, is it a government?

The obvious follow up question is that if these phenomena only differ in size of membership, what makes one "legitimate" and the other not? Is there such a thing as a legitimate government and a legitimate religion, or are they all just another level of street gangs and cults?

For governments anyway we have an answer. Sure at some level the organization of hard force in the form of police is subject to the same practical considerations and constraints as the organization of hard force in the form of a gang, and will thus take a superficially similar form, but a gang does not give you a day in court nor does it set up a system of fair and impartial trials nor does it let you vote on its rules. A gang is fundamentally self serving. It exists for the enrichment of itself at the expense of those outside of it. A democracy, in contrast, directly contradicts the premise "the leader is above the law." So there is such a thing as a "legitimate government".

Existence and uniqueness of "legitimate religion" is left as an exercise for the reader.

(btw Trumpism is absolutely a cult. Just look at it, they had to invent the word "Trumpism" to describe the phenomena because it did not fit any other defined political philosophy and the only constant non-contradictory belief it has is loyalty to Trump himself. Seriously, try defining Trumpism in some way other than "agreeing with what Trump says no matter what." Its impossible. That's a cult.)

Just curious: do you think that any government where the leader is not above the law is legitimate?

Legitimacy is more often a spectrum than a binary thing. Rule of law, by which I mean the law itself is the ruler and not the person enforcing it, is a necessary but not sufficient condition. The essence of what we are calling legitimacy is that the government derives its authority from something other than physical force. In a situation like you described, where the ruler themselves isn't personally above the law, the law itself might still derive from "because some authority said so".

For example, you could have a case like China which up until recently had rulers that stepped down every 5 years. I'd call that a textbook example of the rulers themselves being subject to the rules, and indeed regular peaceful transfers of power is something rarely ever seen outside of Democracies. In spite of having some amount of rule of law, China doesn't quite match the way I've defined legitimacy, because the laws of China themselves are ultimately derived from "because the party said so". So the legitimacy of the laws rest on the legitimacy of the communist party. Where does that legitimacy come from? The short answer is "Chinese standard of living better keep trending upwards forever."