Especially Luffy and Sanji, all Luffy does is goof around or eat. Sanji is constantly trying to impress Nami and Robin. Plus Sanji doesn't seem like the type of guy who spends his free time training. Zoro is always training so that i understand.
Also, Crocodile aswell. He lost to pre-gears Luffy in alabasta and then at MF he's suddenly fighting guys like Jozu, DD and Mihawk. How can someone get stronger from sitting in a jail cell?
1. After ever major battle thy get stronger.
Example: when luffy fought Lucci, he was only able to use 2-3 gear secondo because his body couldn't handle the pressure.
After that fight, he fought Moria and showed that he developed the endurance to use gear secondo whenever he wants.
2. Crocodile has been fighting Haki users before he met luffy.
The only reason why he lost was because he was inside a cave(no flying), damp(no more intangibility), and Luffy had blood on his hand.
Logias aren't physically strong, but luffy had the strength to ko a giant sea king.
Even when Akoji let luffy punch him, he was able to push him back a bit.
When luffy hit smoker by an accident, he sent ace and smoker flying.
Crocodile went 3 rounds of sea king smashing punches and win 2/3 of the fight.
Even Luffy's punches didn't knock him out, he had to punch him throufht the ground and up/or inbetween 2 buildings to knock him out.
So after x amount of hits, crocodile fainted.
Somewhere even suckerpunches...