I know that right now it looks like Zoro is much stronger than Sanji, but bear with me. This theory stems from something that happened lot of time ago, precisely during the Little Garden Arc.
In the beginning of the arc, Zoro and Sanji both went hunting and ended up arguing about who got the biggest prey while they were pretty much the same size. Fast forward at the end of the arc, it is revelead Dorry and Brogy started arguing for the same reason – who killed the biggest Sea King – and from then they fought countless duels without ever finding a winner.
This is Oda’s concept of rivals. Just like Whitebeard and Roger or Roger and Garp fought countless times without nobody ever winning.
Till the timeskip, this holds perfectly. They are always arguing about their bounties and they almost always battle similar strenght opponents. To explain my point let’s look at what Oda has given us.
Firstly, Sanji and Zoro instantly take a disliking with each other with a kinda love hate rivalry with one constantly trying to one up the other. This back in forth would be alot less comedic and meaningful if Zoro was on a higher tier than Sanji. It even translates to their enemies who all share similar relationships to their SH counterparts. Mr 1 and 2, Kaku and Jabra, etc…
Secondly, there’s the fact of their abilities. Zoro has greater physical strength, durability, and Armament Haki. But Sanji has superior speed, Observation Haki and is smarter. They each are Masters in their own respective styles of combat, hinting that if they fought a victory between the 2 would be highly circumstantial. If Zoro hits Sanji it would devastating, but if Sanji can avoid Zoro there’s really nothing he could do to end the fight.
Lot of fans claim that Zoro is now stronger and the gap has increased since the New World but consider this……have they been fighting opponents on the same level? The answer is NO!
Vergo could murder Monet and Doffy is obviously stronger than Pika. Most of Zoro opponents have either been severely weaker or ran from him where as Sanji had legit hard to fight opponents.
Both had hints of their next power ups showcased at Wano: Sanji’s raid suit and Zoro’s black blades.
So I feel like people are quick to jump on the whole Zoro is waaay stronger wagon when there is no info to support that as a fact.
*Theory by Spidey-Jackson