Resolution, frame rate, encoding rate. The higher for each parameter, the larger the file size will get. 8K 60fps files with high bit-rate encoding, as you can imagine, get rather larger. 15GB for 25-30 minutes isn't out of the ordinary.s there any difference in the files or are these just size choices?
And don't forget, that 8K is really 4K-per-eye, not 8K-per-eye.