Bounties aren't necessarily a reflection of threat or ability, just deeds. Ener's bounty wouldn't be 500,000,000 in the Blue Seas for sitting on his arse; given what Raigoh can do, Oda's basically telling us he'd be vaporizing islands left and right. I mean, Buster Call had to go full guns to level Enies Lobby and Ener could have accomplished the same, no, much more with just a little electricity.
As Bellamy said, someone with a 100,000,000 had to have done terrifying stuff in the past to earn such a huge mark; we can assume de Flamingo is the same.
So, looking at what Zoro's accomplished…his bounty is actually inflated more than it should be. Rather than 120,000,000, Zoro should only have 100,000,000 since he didn't really do that much more than Sanji.
I would agree, though, that Sanji has not, nor ever will enter the domain of power that Zoro and Luffy live in. Even if he's gotten stronger, there are physical limitations to his body that anchor him within the realm of "rationality" as far as manga is concerned. His other good "traits" prevent him from amplifying one particular trait (id est, for balancing reasons Sanji's intelligence makes him less powerful than Zoro).
Robin's bounty increase was strange.