I'll have a go at answering this one, please correct me if wrong or add what I've missed. Full length sizing bumps the entire case length back to original calibre diameter, it also bumps the shoulder back too. It does nothing to size the actual length of the case back to max case length for each given calibre, this is why you need to trim to correct size after either neck or FL sizing as the case "grows" when fired, then inside and outside neck chamfer.
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