Post-timeskip, the bounties of the top members of kaidou and big mom has been revelealed. Katakuri is the highest right now with 1.57B, and it can be assume that he is the strongest member in Big Mom Pirates. Meanwhile, Jack has 1B bounty and he is probably the third strongest member of Beast Pirates. The other two Disasters will surely has a very high amount of bounty.
Now my point is that Ace's bounty is only 500M! He is the second commander in Whitebeard Pirates and his bounty is lower than Cracker! Then, does Marco have a bounty that could rival the strongest Kaido's Disaster? That would make the gap between Marco and Ace too big.