Higher bounties because they've been Yonkos a lot longer. Doesn't automatically mean they're stronger or in a position where they'd be able to defeat Shanks 1v1. We don't know enough to come to that conclusion yet. Kaido says that Haki is why Roger was so strong and why he was able to become PK with no DF. Shanks doesn't have a DF it seems, so I would bet money that his Haki is so powerful that it was the reason he became a Yonko and also how he was able to stop Kaido from going to Marineford. His Haki could be so powerful that he could very well be the closest to Roger in strength. The Navy respects him, the Admirals fear him, Blackbeard feared him, and Kaido has put him in the same conversation as Rocks, Whitebeard, & Roger when reminiscing. Shanks has to be hax. I think being so young and being so close in bounty already with Kaido and BM is more of a testament to his strength as opposed to how much weaker he could be compared to them.