in default.css change

wiki-content li {
width: 700px;
}

by

wiki-content li {
width: 100%;
}

This is by design to increase readability. Too long lines can make it hard for you to skip to the next line without losing focus.

A quite good explanation can be found here (my first google hit on the topic):
http://baymard.com/blog/line-length-readability

Best regards
Jan