I think Sci-Hub is the opposite since 1 DOI = 1 PDF in its canonical form (straight from the publisher) so neither duplication nor low-quality is the case.
It does depend on when the work was published. Pre-digital works scanned in without OCR can be larger in size. That's typically works from the 1980s and before.
Given the explosion of scientific publishing, that's likely a small fraction of the archive by work though it may be significant in terms of storage.
