Just looking now.. Mice have similar brain/body-mass ratio as humans. There is no strong (or visible) distinction in brain size between carnivores and herbivores in these comparisons:
https://en.wikipedia.org/wiki/Brain-to-body_mass_ratio#Compa...
https://en.wikipedia.org/wiki/Brain-to-body_mass_ratio#Compa...
> This phenomenon can be described by an equation of the form E = CS^r, where E and S are brain and body weights, r a constant that depends on animal family (but close to 2/3 in many vertebrates