I would think so, yes. I checked the size chart at Petsmart.com and the XL was listed as fitting a large dog like a Mastiff so I would imagine that the XXL would fit a boxer.
According to the Petsmart size chart for Top Paw doggy wear, the XL fits large breed dogs. I imagine that an XXL would fit a very large dog like a Great Dane perhaps.
Our dog was part black lab and weighed about 75-80 pounds. He never did wear this though. I had purchased it and then forgotten about it. By the time I found it again, our dog had passed away.