I've heard some people claim that it's not necessarily the brain/body size ratio that determines intelligence, but the ability to manipulate the world around you. An octopus has 8 appendages, a dolphin has none (maybe its snout and tail could count as two, but are not nearly as efficient as an octupus's arm).
I've never seen any research that backs this up, but I'd love to read some more about these claims. If anyone cares to elaborate?
And you'd be surprised about how little we know about animal intelligence. You should look into the psychology professor Irene Pepperberg and her parrot Alex (Source: https://en.m.wikipedia.org/wiki/Irene_Pepperberg). Fascinating.