There are undoubtedly several factors involved in what you are hearing. You seem to have identified most already. Your own expectations are probably the biggest. Blue Jeans uses Belden wire, which is definitely good stuff. As you correctly pointed out, the gauge has changed although that is probably not a big deal for the length of cable involved. If you hear a change in volume realize that louder always wins every comparison. My philosophy in these matters, though, is why ask why? Enjoy!
Breaking in Blue Jean/Canare 4S11G speaker cable
I recently bought a set of 4 of these cables, 8 foot locking banana at amp end, spades at speaker end. I bi-amp my speakers with two identical SS amps. I had been using 12 gauge Signet wire with Nakamichi bananas for the last 20 years or so. Listened to an HDCD Mannheim Steamroller, 25 year celebration Cd on Denon player that is HDCD compatible. I listened first with the Signet wires then swapped out to the new Blue Jean cables.
I purposely left the volume control on my preamp untouched between this swap. It's not quite a blind test but I very clearly heard improvement in SQ. It isn't night and day but it is improved. The bass seemed more full, maybe a tad tighter. Mids maybe not much different, they were pretty good before. Highs seemed overall a bit cleaner and clearer. Some say there is a long break in process to these cables. I'm impressed already and pleased with what I'm hearing.
One thought is that I went from 10 foot, 12 gauge single/stranded runs to double runs of 8 foot 14g which is equal to 11g. The combination of slightly shorter run and drop in gauge, could lower the resistance (should theoretically) and maybe I was getting a 1 or 2db gain, which can seem as an "improvement". Not sure but it sounds better and it isn't just because I want it to. It is clearly at least somewhat better. Shout out to BJ/Canare.
I won't jump up and down and proclaim it's the best or whatever but it is an improvement over stock 12g speaker wire for sure.