@cheese4u - In addition to what everyone else said (I told you so
), you're directly correlating bounty to strength and it doesn't work that way. Bounty isn't an accurate reflection of one's fighting power; it's primarily reflects on the person's threat level.
Blackbeard became a warlord with a bounty of 0, but obviously that number doesn't reflect his monstrous strength. Chopper is most certainly not just a pet worth a measly 50; he defeated a CP9 agent by himself which is a remarkable feat. Zoro and Sanji are comparable to Luffy in fighting strength (they are known as the monster trio for a reason), but clearly their respective bounties of 120 million and 77 million as opposed to 300 million (now 400 million) don't accurately reflect that. So, as you can see, the bounty=strength system doesn't work.
As for Croc, Luffy only won due to circumstantial plot (much like he did against Enel and Moria, as well). And that makes perfect sense, seeing as how Croc is a shichibukai class pirate, whereas Luffy was just a rookie. Oda made it a point to emphasize the difference in caliber between the two levels. Even at Impel Down, Croc was treated as the same caliber as Ace and Jimbei; they all underwent the boiling baptism without even flinching and were all stationed on Level 6. Pre-time skip, Luffy wasn't on their level (as in strength, if you will). Finally, the war at Marineford showed us what Croc is like when he doesn't underestimate anyone (and for good reason seeing as how the battlefield was full of elites); he can hang with the big boys.
Croc was one of 7 elite pirates chosen to work for the WG. They deemed him worthy of shichibukai status, and thus that is a testament to him always being very strong. The WG wouldn't recruit someone weaker than a rookie, especially since the Shichibukai were formed to counterbalance the Yonkou.