Previously I always thought that was quite a high bounty for him. He left on his journey three years before Luffy so hadn't been a pirate for long and as far as we know he never did anything as threatening to the WG as what Luffy did at Enies Lobby (freeing Robin, buster call, cp9 - all a big deal individually) and wasn't a nutjob like Kidd so his bounty wouuld really have no reason to go past about 300M and if you look at the worst generation, apart from Kidd and Luffy, Trafalgar D Law's bounty was the highest and pre-skip was only 200M (440M post-skip). 550M could be justified by being Roger's musko son but Luffy was Dragon's son, so it's kind of even there anyway, so most likely it's down to being in WB's crew. He didn't seem to be all that strong compared to otjer yonkou or shichibukai (was easily caught by Blackbeard, who clearly wasn't all that yet, as we saw in Impel Down). I think he was 2nd commander due to being seen as having potential to be the pirate king. If his bounty was somewhere around 200M pre-WB, then getting up to 550M is quite a big jump just for being a WB commander for about two years. A captain like Kidd or Law only hit around 450M after the skip, which was a 200-250M increase. Getting an increase of around 350M in two years (which is probably about how long Ace was in WB's crew) as a subordinate would normally be totally absurd, so I'd say he definitely did get a very sizeable bounty boost of at least 150M, due to being in WB's crew, so his bounty makes sense.
