How is typesetting manga different from typesetting American comics?
US comics tend to use speech balloons with a horizontal format, whereas Japanese comics use speech balloons with a vertical format (because of vertical writing). Manga balloons, in Japanese form, and in well-lettered English translations – use a LOT more white space within the speech balloon. A ‘typical’ US comic will use horizontal white space equal to one or 2 characters. A ‘typical’ manga will use horizontal white space equal to the longest line of text. Shoujo manga will sometimes have a little more white space, and Shonen manga will sometimes have a little less, but they will generally have more than US comics.
US comics tend to rely on the order of panels to guide the reader through the page and through the story. Manga is much more likely to use the word balloons to guide you through the page and the story, which means the speech balloons are more likely to cross the panel borders.