The size of the file will also depend on the video encoder that is used and the content of the video itself (50mins of a visual novel game would be much smaller than 50mins of an first person shooter, for example).
1.6 - 2.2 GB for a 50 minute recording is actually quite small and definitely at the low end in terms of what you should expect. But 26 - 28 GB is much bigger than anything we're seeing. As a rough guide, when we record for that length of time at 1080p, 60 fps, we tend to get anything from 5 - 15 GB depending on the game.
All we can suggest is to use the Send a Problem Report option on the Support tab to send us a report next time you get a huge file and we can check the log files to see if there is anything unusual going on.
Next time this happens, do not leave it on that "Stopping recording..." screen for more than a minute before closing it down. Then restart the app and use the Send a Problem Report option on the Support tab to send us your log files to check. (If you leave it on that screen for too long, your log files will grow to large and we won't receive anything)