I tried to create a user, but there was an error about it being restricted. Therefore, my best guess is that there is padding that you can remove. For instance the ".sourcecoast .btn" style has a padding of 16px 22px. You can try decreasing this (however, all buttons in SCLogin would be affected) or create a new style targeting just the logout button and user menu buttons specifically for the padding - "#scLogoutButton .btn" and ".scuser-menu .btn" for the identifiers
Either way, I'd suggest that you try using Developer Tools built into your browser to play around with and test out any quick CSS changes (Firefox and Chrome have this)
-Melissa