Is Kiroumaru really a bakenezumi?
Think of Queerats as humans (WINK WINK). A person can be Asian, Caucasian, etc. So, there are many different types of Queerats too; some of them have horns, some have smaller teeth and (in this case) some of them are tall.

HOWEVER. There IS a speculation regarding Kiroumaru's height that involves the origin of the Queerats. It's a last ep spoiler:


   Since Queerats are from non-PK humans, Kiroumaru's height might come from this. It might mean that he or his race didn't "evolve" properly into Queerats. Or, Kiroumaru's race might be tall in the first place. That is, when they were still humans, they were tall. So, when they turned into Queerats, they're tall too.


Hope that helps you :D