Guardians of the Galaxy Vol. 2 is a little bit longer than its predecessor.
The first Guardians of the Galaxy movie was almost exactly two hours long. But as is often the case with fantasy/sci-fi movies, the producers have used a sequel to expand the universe. Because Guardians of the Galaxy has comic source material there are a wide variety of characters for the writers to draw from, even without original creations of their own. The addition of characters such as Mantis and Ego basically confirm that Guardians of the Galaxy Vol. 2 will do just that, which is why it has a runtime of 136 minutes.
While the initial reviews of the movie have been mostly positive, the public will have to wait until May 5 to judge for themselves. The danger is that additional characters could make the film overcrowded and mean less time with those the audience cares about. Who knows, though?
One of the advantages of using stories based on written material (comics or novels) is that the creators or authors did not have to deal with things like budgets. This means that the comics will inevitably have more characters and storylines than a single movie (or even a series of movies) could handle. This means that franchises with multiple movies can expand while still having something to work off of. Also they can pick and choose stories they already know to be fairly popular.
Next: Who stars in Guardians of the Galaxy Vol. 2?
The real question comes in the execution and the creative thinking behind it. The first Guardians of the Galaxy movie was an overwhelming success with both critics and fans. As such its not surprising that Marvel would expand the palette of the series. They could even be setting up the possibility of spin-offs and independent movies set within the same universe. Marvel already has one mega-franchise with the Avengers movies but missed out on a second by selling the movie rights to the X-Men before becoming a film company. It could look to Guardians of the Galaxy to become another series on that level.