I think their bounties are okay when you look at it this way:
Dragon is a secretive man, wouldn't it make sense his revo army makes sure they don't show any connection to him?
So I think everyone got their bounties while being lowkey. Remember they're all split across the seas
& after the incident at the reverie, I can assure you their bounties will go up. It is possible they earned those bounties while keeping their connection to dragon and the revolutionary army private.
Either they got those bounties by being lowkey, or after the stunt they pulled that is their bounties off rip & only sabo gets a raise from 602.
Considering they don't know exactly where they rank in dragons army, they probably give them sizeable numbers without overduing it because they don't even know what dragon is plotting.
Even sabo was shocked to see he was found out in dressrosa.
Koala & Hack stayed hidden too so its possible even they don't have bounties yet. Remember when they just found out Buggy was a part of Roger's crew after Impel down?
I think they are okay. It's the fans who mistook the the RA to be a yonko like force while it's clearly not.
The commanders all have bounties comparable to shichibukai and worst gen members. Those are pretty much the highest figures outside the the YCs and Yonko captains.
Kuma should of been definitely on that level too, too bad he might be out forever at this stage. I would like to acknowledge what happened to him at last. The situation is not quite clear if I recall correctly.
Only justified if Dragon's bounty is ridiculous. But I don't think it is the case, he probably has higher bounty than Roger. So it is unacceptable that not even the number 2 in command have at least a billion bounty.
Just another proofs that Oda writes random numbers when it comes to bounties. Only Warlord have a nice excuse for having low bounties.