And I'm not sure how you can reduce a label to 80% and have it still work, though I'm admittedly somewhat ignorant on barcode standards. Do the labels still even scan?
I do know a thing or two about barcodes. They're not based on absolute measurements of line thickness, they're proportional. Look at any random half dozen different items with barcodes on the packaging, odds are they'll all be the same standard and there'll be at least three different sizes represented. A barcode reader that can only read one specific size of barcode would be fucking useless.
Therefore, as long as the scaling is fairly precise, you can shrink (or grow) just about any given barcode and still reasonably expect it to scan. Allowing for how precise with small print or how wide an area the average barcode reader can handle, of course.