Bounties are only a representation of threat, not power. There are rare occasions where it's used as a measuring scale, such as with powerful Grand Line Pirates (eg. Shichibukai) when the Straw Hats or another character are going to take them on, but even that's inaccurate, as it's all merely a number to show their threat to the Government. For example, Robin having a 79,000,000 Beli Bounty from Age 8 due to her knowledge of the Poneglyphs.
Overall, it doesn't mean much. For instance, Crocodile's bounty is one of the lowest of the Shichibukai, yet without spoiling anything, there are a few pirates he could hang with who have far higher bounties.