You got it. Jack said something about the 80% rule only applying on the primary side and it's already built in at the panel (14ga wire being on a 15A breaker even though it can carry 20A). I'm not entirely clear on this though.
I've been using 16ga main runs on some small jobs recently (like 6 - 8 fixture jobs) and am going to just start putting a small fuse block in or next to the transformer so I can fuse those wires correctly. While the cost difference isn't that much on one job using 100' of wire, 12ga wire is 2x the cost of 16ga wire so over 10 jobs, it starts to add up.