Well the same applies to Crocodile. Luffy was a no name pirate in paradise he lost 2 times to Croco and then he beat him. One of Luffy’s greatest abilities is his ability to grow stronger even during a fight. The same was said by Rayleigh “the true strength of haki blooms during a fight“. Luffy is growing stronger right now. But just because he beats Kaido doesn’t mean he can beat all the other yonkous easily. And it’s not a couple of days he beats katakuri a few months ago. Which in OP time is a lot. And he has trained, learned ACoA and ACoC. Which is important cuz it shows that kaido only beat him so easily thanks to ACoC and now the ground is event out. But Luffy keeps growing during the fight. The Luffy right now (chapter 1034) is still growing. At the end of the fight will be even stronger.
And his bounty doesn’t just reflect his win over kaido. It reflects the downfall of a yonkou by another pirate. Which is unheard of. It reflects the fact that he has 3 RPG, a person who can read the PG, and is the closest to becoming PK since Roger. All of this added up Garantie a bounty of 5B in my opinion but I can obviously be wrong.
Crocodile's two wins over Luffy came by way of Luffy not understanding Croc's advantage = sand power taking away the water inside Luffy. Once that advantage wasn't there anymore, Luffy knocked him out as Crocodile wasn't necessarily more powerful than Luffy.
The sheer power difference between Kaido (or Big Mom) and Luffy was pictured as immense and still should be. The storyline excuse of using a power (ACoC) for the first time an hour back and immediately being able to be on the same level as a guy who did it for decades seems very weird. Kaido fought as well in past decades against top level guys, probably more than Luffy.
I would have no problem with Luffy getting a bounty of 5 billion after beating Kaido, it's quite close to my 4.7b, but I simply criticize the whole topic from a storyline perspective as it's a deleopment that is far too rushed. At this point after being declared by media as being the fifth Yonko he should now prove that he's starting to close the gap, but not completely doing so.
The Katakuri fight wasn't many months ago, maybe one month. The story we've seen in One Piece does actually happen in a very short amount of time with the two year time-skip being a gigantic time period in comparison. It just feels like many months :D