One reason why Game DVR may record lass than 5 mins or 10 if you set it to those settings. Seems Like it has its own built in safe mode. I tried to record a rocket league match in 60 fps and High quality and the quality is there. But it only recorded 2:34 of an entire match. I currently have it set to 5 mins. Looks like there's no way around this.

http://support.xbox.com/en-US/games/troubleshooting/recording-problems-with-gamedvr

A clip is shorter than expected

Under most circumstances, Game DVR allows the creation of clips up to 5 minutes in length. At times, you may find that your clip is shorter than anticipated. There are a few different reasons why this may happen:

Time spent outside the game is removed from the clip. For example, if you back out of the game to the Home screen while recording, the time at the Home screen will not make it into the clip. Publishers may disable the recording of certain game segments for several reasons that may include, but are not limited to: Potential game spoilers. Views of a player or players in the game seen through their Kinect. Game DVR records clips at 720p, 30 frames per second. For most games, this resolution will allow a clip of approximately 5 minutes. Clips with significant graphic intensity or action may be shorter, as duration is sacrificed to maintain a high-quality recording.