I'd imagine O-5 to O-6 is the biggest dropoff, especially since O-5 pay is really good and from that point on you don't get to actually do your job anymore. Once you hit full bird you're well into the political sphere of the officer corps and just need to focus on suckin the right dicks to get your stars.
edit: holy fuck I'm way wrong
At the end of the fiscal year of 2024, it is estimated that there will be 15 Generals serving the United States Army, and a total of 354,948 enlisted personnel.
www.statista.com
dont know how accurate that is but yeah, dropoff from over 3600 to just over 100 going from O-6 to O-7. i imagine the air force ranks are WAY more of a dropoff going from O-4 to O-5 to O-6 than the army due to no warrants. take all those warrants in the army and distribute them and the numbers would inflate a LOT for the O-1 to O-5 ranks.