You are looking at it the wrong and your wording about Zoro's contribution makes you look like a Zoro hater.
The thing about the recent bounties issued is that Luffy, Kid and Law were given 3 bil each as they were together seen as a bigger threat than BM and Kaido whose combined bounty was less than 9 bil. So if those three can be seen as a greater threat because they defeated the two Yonko and hence receive a greater overall bounty than Sanji and Zoro must also be given bounties that surpass the threats that they defeated (as long as we are speculating).