I think the bounties make sense when you look at it in terms of accomplishment and not strength.
Luffy did 1v1 kaido... after kaido fought the scabbards, zoro, kid, law, killer, and yamato, all while holding up onigashima. Luffy also lost multiple times.
Kid and Law flat out 2v1ed big mom. The bombs helped, but the bombs were only a factor because of a situation Kid and Law forced linlin into. Law created a hole, Kid pushed her down it, had she not grabbed the bombs, Linlin would have still fallen into the earth.
Luffy's should be a bit higher and kid and law's, a bit lower. However, I take this as a message that Luffy didn't truly surpass kaido.
That makes sense. This way akainu, shanks, and teach can still be important without massive powercreep.