Kind of expanding on Matt's theme, and seeing the latest square compression & stuff sacks in the Piragis catalog.

You compress a normal round compression bag till you've got a round rock,,,throw it in your pack,,,and add your small items in the voids between the round rocks you compressed. Verses packing slightly compressed bags that smash into the voids and stuffing small items till it all fits.

I know from packing hard sided "Blue Barrels" on river trips you can get more into a blue barrel if everything gives some, i.e. stuffing loose stuff into the barrel works best.

Think about the total system. Not just how small one bag will get, but how it all works together.

Personally I like Matt's suggestion that a large compression bag is easier to stuff something into, and only meant to be compressed slightly. I don't know about how it damages a down bag. I'm still using a Eddie Bauer down bag from ~1970. When they only had a Seattle & Chicago store. Proof again that buying the best you can afford is cheaper in the long run.