If you are referring to your forum index's sidebar, those usernames have a very specific .sidebar .username class which would not apply to .vw-list-contributors because it is not in the sidebar.
The usernames in the main forum list (last post column) are inheriting the font size defined for the .nodeLastPost column, which is not relevant to .vw-list-contributors, but only to container-like node-list entries, like a list of forums or list of wiki areas. It is not defined for the .username class itself. If you set a font-size for the .username class specifically, then it will be applied everywhere the .username class is used (including .vw-list-contributors), unless it is overridden by something more specific like .sidebar .username.