No i cant say if a bitrate of say 8 mbit would only take up half of the same recording made with 16mbit, i don't think it correlate just like that, but a lower bitrate for sure mean a smaller file.
I sort of look at bitrate like a multiplier, the higher the better, but higher bitrates also mean bigger files, i have run custom firmware's with faster bitrates and that made my file size for a 3 minute 1080p recording jump to 1 gigabyte instead of the normal 300 or so megabytes.
For me the main thing with parking mode are to "tag" and secure the moment something happen, so constantly recording i will prefer not to deal with cuz it mean 2 things:
1 : you will have to visually detect the damage, and risk driving for days before you notice that dent on the back of your car.
2: even if you do detect yourself a event have occurred, then you have to go over XXX minutes of regular recordings to find the moment,,,,,, which i would of course also happily do is someone rammed my car and left.
I am hoping cameras now are say constantly recording low bitrate, and then upon a event change to full bitrate parking and save that, this way i have the G sensor event as a time stamp and high bitrate recording after that, and leading up to the event i have low bitrate regular recording, that will be easy to find with the time stamp on the G sensor event.