User blog comment:Kaido King of the Beasts/Chapter 855: The Big Sleep/@comment-7890302-20170209132411/@comment-53539-20170211150506

It maybe because of how large the crew is, the overall bounties were spread thinly across the entire crew... Collectively with Big Mom they may end up having a huge total amount, but individually the regular crew may have quite a divide among them.

And don't forget also, a element of said power is made up of alliances between pirates that support a yonkou collectively and can be called upon if need be. so once again, Big Mom's power can be split more thinly throughout her alliance as well as her crew for this reason.

Regardless, the total power would still make her a force to be dealing with. You upset her and yo get her family + their families after you. Thats a massive force to deal with. Remember how many children she has and how many were married off to get alliances for power here, that consists of everything including a Fishman crew?

Another possible explanation is that Big Mom's influence has impacted the amount and she has manged to obtain the power to keep her crews bounties lower then they should be. If Sanji's bounty can have "alive" only on it because of influence, its possible others can do the same. Considering a lower bounty gets less attention and is perceived as less of a threat, is may result in people going into a fight with a crew member of hers and not establishing a strong guard against her crew member, resulting in them not doing enough to fight back and making for an easier victory.

Thirdly, there is also the possibility that Big Mom, though is a Yonkou, is the weakest of the 4 and it just scales up from her. Just as not all the Shichibukai have equal strength, its likely so true for the Yonkou. Don't forget its the combine power of the 4 that keeps each other in check, it means the strongest can't progress forward without giving up more to the weakest of the 4 and allowing them to become stronger in the process. This is how the Yonkou have ended up coming to such a stalemate. the death of WB ended up with the loss of Fishman Island to Big Mom, for example, if Big Mom is the weakest of the 4 then the strongest had lost a piece of power to the weakest, making Big Mon slightly more powerful then she was before in the process.

Either way, the bounties can be plausible and may not have to be all that high.