The climax is not the biggest scene, it is the decisive one. Spectacle is frequently placed there and frequently is not it: if the outcome would be the same without the protagonist’s choice, the explosion is a set piece and the actual climax is somewhere else, probably a conversation.
It has to be the obligatory scene — the one the audience has been promised by everything preceding it. Denying it is possible but expensive; the audience will accept a different answer to the question, and will not accept the question being dropped.
The most common commercial failure is a climax that resolves an external problem while the internal one is left untouched, which produces the ending that is technically complete and emotionally weightless.
What is the climax of a story?
The scene where the dramatic question is answered by a decision the protagonist makes. Not necessarily the largest scene — the decisive one.
How do you know if your climax is in the right place?
Ask whether the outcome would change if the protagonist did nothing. If it would not, the scene is a set piece and the real climax is elsewhere.