Oda gave his justification for this. He said the world government doesn't know exactly how strong each person is so their bounties are tied to their captains. Which makes sense, but as a hype tool it's moot.
Maybe to Katakuri, but Jack? Lmfao. Luffy beat Crocodile and Enel without Gear 2/Gear 3 who are solidly stronger than Jack. Crocodile outshone every freaking Whitebeard commander on Marineford. He did better against Akainu than Marco & Vista combined. Not to mention he made Blackbeard scream in pain with 1 basic hit who was > Jack too.
300 mill is fine for lower Veteran level like Daifuku, Oven, which I believe MF Luffy was close to. It's just the gap from that, to giving Perospero 700 million was the dumb part.
The "real" bounties should have been something like 300 million for Daifuku, Oven, 400 million for Perospero, 500 million for Cracker/Smoothie/Jack/Queen, 600 million for King, 700 million for Katakuri.
That "nobody" was given Conqueror's Haki which only the Pirate King's right hand has been given so far and has been undefeated throughout his pirating career.