He's physical strength didn't change significantly.
Throughout the sagas the strawhats develop in one aspect after another.
When luffy had access to his gears, he stop relying on his standard form and use his gears to fight strong opponents
I'm not saying his strength w/o gears stopped, it just not as astonishing rate as you think it is...
Ex: before luffy pushed two huge buildings/castles, which is outrageous but in time skip he takes out a giant in the colosseum in gears 2. <----shouldn't of been needed.
So he did get stronger between crocodile fight to the cp9, but it wasn't a significant leap of strength that you would say Luffy's strength pales in comparison to the lobby episode.
In jail or impel down, they torture the inmates there.
What I'm saying us that crocodile build some durable/resistance feat in the prison.
And judging from the beatings he got from luffy in comparison to how he tanked some in the marineford.......wouldn't you agree?
I think we both agree that Luffy atleast did grow better than Crocodile post Alabasta, not counting the gears. Crocodile becoming more durable is a good point and I agree with it. What I don't particulary don't agree with is that Crocodile's feat against Jozu is not legit because that is a major jump in power.