This is the #5 and #10 US airlines combining, with 12 total routes they both operate on, only 2 of which have no other competitors on it. The #5 airline is ~25% the size of #4, and #10 is ~25% the size of #5.

The DOJ literally just blocked a merger between the #6 and 7 airlines to protect competition and prevent market consolidation, and won a suit to block a comprehensive alliance (where they could coordinate schedules and pricing) between #1 and #6. I'm going with 2023 enplaned pax count as the size ranking but I am sure everyone gets the idea of the relativity here.