What is intelligence? What is it's basis?
Humans consider themselves the height of earthly intelligence, but they do not know how or why. Some animals perform tasks we consider intelligent, arguably some plants, and despite having seeming to have the proper biology, human intellect varies extensively from person to person.
We don't know nearly enough about the subject to be able to say why anything is intelligent at all, much less more or less than some other species.
However, crappy freshman bio answer:
Yeerks have a different neurobiology (smaller, but more cells and/or more efficient neural arrangements) allowing them to have higher cognitive function than similar sized, or even larger earth animals