Use 'gargantuan' to highlight the exceptional size or scale of something, more than 'big' or 'large' would. The library had a gargantuan collection of books.
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Visual Impact
'Gargantuan' often creates a strong visual impression, suggesting something is impressively large or overwhelming. They stood before the gargantuan statue in awe.
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Not Literal
Sometimes 'gargantuan' is used figuratively to describe non-physical things like tasks or achievements. She had the gargantuan task of organizing the event singlehandedly.