Um, so, you are burning HD Blu-Ray with an HD file. That makes sense. Not sure what the issue is. 720 is full HD. 1080 is full HD. HD comes in many flavors.
I'm going to assume you shot in 1080. Be sure your Project timeline is 1080. Select it in the Browser, go to the Inspector, in the Info section click the "Modify Settings" button. Once footage is in a Timeline, you can change the Frame Size (which you want to do), but not the Frame Rate.
I think that is what you're looking for. Change the Project timeline to 1080 to match or footage.
Also, verify your original footage was actually shot in 1080.