You probably have a high-definition screen (bigger than FD). This method of displaying the message list was probably coded before such monitors existed, so some dimensions have been hard-coded into the program. No-one has come up with a solution yet - adjusting the screen DPI upwards has too many negative effects elsewhere for it to be usable.
If this is your situation, the workaround is to select One-line view under Message list on the View tab.