Also bear in mind that they have different publishing strategies. The small Japanese volumes are usually a collection of chapters that were originally published one chapter a time in magazines that publish several different ongoing stories. The chapters for one story might be published once a week, or once a month, or possibly once every few months. Because the magazine has several different stories in each issue, and has several different stories to rotate through, the creator does not necessarily need to complete a new chapter every week or month. Even if they are creating it at that rate, the chapters tend to be shorter than a US comic issue. When they have enough chapters completed, and if the story is popular enough to readers of the magazine, they gather up all the published chapters and release them collected in the volume form.
On the their hand, most American comics are published on a monthly basis. Similarly to the Japanese comics, very popular story lines will be gathered into graphic novels.
Another difference is the amount of finished detail that goes into the art. The Japanese manga magazines all publish in black and white only, with color only being on the cover. While there are American comics that are not colored, the ones from the major publishers are all colored, and including coloring in the production process means more time is required for each issue.