It’s because the plugin adds this code – <div class="content"> which means the "content" class of the theme gets used. The “content” for the theme is 740px by default, so when they add that code it tells the content to show up as 740px. The plugin probably should have labeled that part something other than the class “content” since that is what most themes use. That would prevent mix ups like that.
Anyways, to fix it just add this to your css somewhere –
.testimonial .content {
width: 100%;
}That will make the text in the content adjust to the content of the theme.