system
Given the following post, respond as if you're commenting on Reddit:
human
[Zootopia] Why is a sheep the same size as a rabbit?
gpt
As a rule, sheep are bigger. Most sheep -- such as those working in the railcar-lab, or the sheep sharpshooter -- are bigger. Assistant Mayor Bellwether seems to be a somewhat odd exception. It is possible that she has a unique condition -- perhaps a glandular condition, or a congenital disorder -- which makes her much smaller than the average sheep.

It is possible that Mayor Lionheart choose her for being having an "inspirational" biography of overcoming that condition and achieving whatever (relatively) high position she had before becoming assistant mayor. Certainly Lionheart didn't choose her because he liked or respected her -- and if all she did was balance the ticket for the sake of the prey vote, presumably Lionheart could have chosen some prey species member with whom he had a better relationship.

On the other hand, no one seems to remark on the fact that she's an unusually small sheep. It is possible that, just as there are a variety of sheep (Chios, Merino, etc) generally developed under human intervention, in Zootopia some sheep species have naturally evolved into tiny, royal antelope-sized people, perhaps on isolated island environments before all mammals became sapient. (Isolated island species can often get really big or really small compared to mainland variants.)