Bounty has very little to do with strength. While a lot of things can influence bounty, there are 2 main things that affect bounty, and out of those 2, there's 1 factor that influences bounty the most.
The greatest influence is the amount of crimes committed. No one gets a bounty of 500m or 1 Billion just like that. They must have committed a lot of crimes to begin with that are known to the Government.
Boa Hancock at the age of 18 acquired a bounty of 80 million after 1 venture out to sea; Robin acquired a bounty of 79 for simply escaping the marines at the age of 8 because of her ability to read Poneglyph. We can't say that an 8 year old Robin is as strong as an 18 year old Boa Hancock, so strength really doesn't influence bounty that much.
For about 20 years, Robin's bounty remained at a value of 79 million until after Enies Lobby where her bounty went up by 1 million. It took 20 or so years for her bounty to to go up by 1 million, why? because she'd not committed any known crimes to the WG's knowledge. have to also note they both earned similar bounties for different reasons. One for probably pillaging and the other for her being an archaeologist meaning, when looking at a bounty, one needs to also have background info of the nature of committed crimes. Can't look at a bounty and simply infer that this character is stronger than that character. There's a lot more to reading bounties.
When we see high bounties, the most we can infer is the character has committed a lot of crimes known to the Government and this Pirate must be quite experienced at the very least.
It took Luffy a lot of crimes to gain a bounty of 500 million, and bare in mind that crime is anything that directly or indirectly shows defiance against the WG and the law of the land.
- Luffy defeated Krieg, Captain Kuro and Arlong before acquiring a bounty of 30m.
- He then brought down a Shichibukai in Alabsata earning him a bounty of 100m.
- Then he defeated Moriah where his bounty was raised to 300m.
- Not long after, punched a Tenryuubito, broke into Impel Down, saved Ace and then rung the Ox Bell which earned him a bounty of 400m.
- Finally, he took down Joker where his bounty once again rose, and to 500m.
5 points, but he's easily committed 10 crimes or more. Just imagine if they knew he'd defeated Enel, his bounty might be higher.
One point I'd like to drive is the Supernova are all Pirates with a bounty of 100m and higher.
Urouge had a bounty of 108m when he reached Sabaody while Luffy had a bounty of 300m; Urouge with a bounty of a 108m was able to almost defeat a Pacifista while Luffy with a 300m bounty was only strong enough to do about the same thing implying their strength levels didn't differ much, only bounty.
Also, think about it. Urouge had a bounty of a 108m while when Luffy had a bounty of 100m which was after defeating Crocodile, he didn't have Gear 2nd or 3rd. The Urouge at Sabaody with a bounty of 108m was far superior to Luffy when his bounty was 100m. There's no way Luffy could have remotely replicated what Urouge did when he had such a relatively low bounty.
Then we see after the time-skip, to our knowledge, Urouge's bounty is still just a 108m and he is actually the most successful of the Supernova. Kidd, Bege, Apoo all failed to defeat snack. Only Urouge was able to do that much despite having the lowest bounty.
So, while we look at the bounties of Perospero, Cracker, and all these commanders, we need to take their years as pirates into accounts. How long have they been Pirates? They're definitely not young, so they'd have had ample opportunity to venture out and make names for themselves to earn themselves a bounty. It takes years to earn such high bounties. Katakuri may have been a Pirate for 20 odd years, or a decade at the very least. A decade to earn a bounty of 1 billion as opposed to someone who earned a bounty of 550m in a mere 3 years shows how impressive Ace's bounty is (as is Sabo's).
Bounties do indicate experience more than anything regarding the individual; it also indicates they've likely committed multiple crimes, but one has to understand the nature of the crime too. Bounties cannot be understood purely on value, but requires context like character history and nature of committed crimes.
Ace was super strong. Even Chinjao believed he was the closest amongst the rookies to become Pirate King and Sengoku also felt similarly which indicates just how much of a threat he really was.