Kallithea is using a large mix of font sizes, big and small, inconsistently. At least, I don't understand the logic.
Examples of illogical font sizes are the repo URLs on the summary page, and the pull requests overview page of a repository.
A good UI has a very limited amount of font sizes in use. Most text will be in the normal size, there will be some exceptions in small font, and things like titles would be in a large size.