To increase something to a very large size; to enlarge or magnify something dramatically.
Compound of 'giant' + 'size', functioning as a verb (to giant-size = to make giant-sized). The word became more common in business and marketing contexts, particularly from the 1950s onward with terms like 'king-size' and 'super-size'.
You've almost certainly heard 'super-size me' (the famous McDonald's phrase), but 'giant-size' was used earlier in advertising—it's how marketing borrowed metaphorical size language to make products sound grander and more appealing.
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