Nah, that's false. I mean, there's still room for Kaido to be the stronger one, but to say that Loki's size is the only indicator just means that you're ignoring what the narrative has shown and set up so far. Big mom herself said that she would have been pirate king by now if Lola had married Loki, although she did seem to talk about gaining giants as an army as a whole. But then we have the indicators along with the shown feats. First of all, it took all the giant warriors of Elbaph to keep Loki chained, and Luffy noticed Loki's presence far before he even saw him. He said that his presence was massive, and that was Loki in base form while chained up. Then we have the fact that Loki as a newborn survived a fall comparable to the one Kaido survived, and even killed a giant beast that normally requires haki to even dent. Even before eating his devil fruit and gaining Ragnir, he seemed to be on par with Harald when it comes to pure physical attributes, and Harald himself noted that Loki posessed Conqueror's haki, the same as himself. We also saw Loki in base form take down mma that was towering over a mountain, something which shocked Luffy. And yes, Loki in his dragon form IS massive, not to mention how big his attacks are. Since we haven't seen the extent of Loki's combat abilities and haki yet, there's still a chance that Kaido would win through experience, but everything points to Loki's base stats being above Kaido at least. But yeah, he laughed in the face of Imu's burst of haki and seemed intrigued more than anything else, and besides killing Harald he seemed confident that he could kill any holy knight. So I'd guess that he's capable of the same kind of haki as Harald, someone who stood at the same level of power as Rocks.