It's because of perspective. Koby destroys an island sized arm. Oda didn't draw it from Koby's POV. He did the same with Zoro when he cut Pica.
And Luffy's trails don't seem to get bigger to me. Those of Ulti vs Luffy were undeniably bigger and funny enough Aokiji vs Garp
Those aren't the ends of trails but the end of the panel. The trails clearly go beyond what's shown on the panel. Again, it's a close-up panel while you got a full view of Coby's attack. And that argument isn't making much sense. What difference does it being drawn in Coby's perspective even make? Clear indications of CoC were drawn when Garp broke the skull.
Again, I won't deny the possibility of it being CoC because the DC is just too insane for a CoA attack. But there's no reason to try and hide it from the readers by not drawing the impact lines if Oda's conveying the fact that Coby's in fact a CoC coating user.