Zoro's scar isn't small at all, it's only overlaid on the scar caused by Oden so it looks smaller than what it actually is.
And if you ask me anti Zoro pirates already know that, but they do this only to troll and bait a little bit
As you can see it starts from the exact intersection of the 2 lines of the old scar "X" and it spans up to Kaido's shoulder and that's a huge scar.
Don't let distract you from the fact that during the weekly release there could be many drawing errors and that Oda is highly inconsistent when it comes to sizes.
So while the old scar had the shape of the letter "X" now Kaido's scar looks like a sword.
For a comparision: if Zoro's scar wasn't overlaid on the Oden's scar it would look like this (more or less):
That's fucking impressive, but there is a reason why Oda chose to overlay Zoro's scar and that is that now Kaido's scar looks like a
sword