Grandiose describes something large and impressive in scale or conception — and, far more often in current use, something whose scale or self-presentation outruns what it can actually deliver.
Both senses are recorded, but they pull in opposite directions. The neutral sense survives mostly in descriptions of architecture, landscape, and music, where grandioso remains a performance direction meaning broad and stately. In ordinary English the pejorative sense dominates: a grandiose plan is not merely large but overreaching, its ambition asserted rather than demonstrated. The reliable tell is that grandiose almost never describes a finished achievement — grand does that work. Grand praises without qualification and grandeur names the quality itself, while grandiose withholds; it is the word for a gesture that would have been grand had it been earned.
English took the word in the nineteenth century from French grandiose, itself from Italian grandioso, from Latin grandis, "great, full-grown" — the same root behind grand and grandeur, which is why the three words sit so close together and why the difference between them is entirely a matter of judgment rather than degree.
The adjective also has a technical life in psychiatry, where the related noun 📝grandiosity is the first diagnostic criterion for Narcissistic Personality Disorder and "grandiose narcissism" names the overt presentation of the disorder, as against its covert counterpart. In that setting the word is descriptive rather than dismissive — a claim about the structure of a person's self-concept, not about the taste of their ambitions.