You have two options:
- Get a BD50 blank disc to burn the ISO file on.
- Use the free software Handbrake to decrease the file size of the video stream, itself–demux the audio/subtitle tracks, first. Then, go back to TSMuxer and load-in the new video stream and the audio/subtitle tracks, making sure you uncheck the video stream of the original file. If the ISO file is still too big for the BD25 blank disc, then go back to Handbrake and make a smaller file size of the video stream. In other words, play with the file size of the video stream and see if the picture quality works best for you before making the ISO file.
Thank you for the advice! I will consider 1 as the much easier, if not cheaper, option. Meanwhile I will look into 2 and see if I can fiddle around to my satisfaction.
I had been under the impression that the source files were already sized to fit a BD25 disc, but I seem to have been mistaken!