I've used a wrench to hanmer in a nail more than once, but that doesn't mean it was the best tool for the job.

It isn't that agile can't be used for something big, but that the design will likelyntun into hard requirements that must be approached certain ways and at thst point you are using agile to do waterfall. If it improves communication earlier in the process (which waterfall does not prohibit) that is great!